A minimal docker baseimage to ease creation of X graphical application containers

This is a docker baseimage that can be used to create containers able to run any X application on a headless server very easily. The application's GUI is accessed through a modern web browser (no installation or configuration needed on the client side) or via any VNC client.

Table of Content

• A minimal docker baseimage to ease creation of X graphical application containers
  • Table of Content
  • Images
    • Content
    • Versioning
    • Tags
  • Getting started
  • Environment Variables
  • Config Directory
  • Ports
  • User/Group IDs
  • Locales
  • Accessing the GUI
  • Security
    • SSVNC
    • Certificates
    • VNC Password
    • DH Parameters
  • Building A Container
    • Selecting Baseimage Tag
    • Referencing Linux User/Group
    • Default Configuration Files
    • Adding/Removing Packages
    • Modifying Files With Sed
    • Modifying Baseimage Content
    • Application's Data
    • $HOME Environment Variable
    • Service Dependencies
    • Service Readiness
    • Log Monitor
      • Monitored Files
      • Notification Definition
      • Notification Backend
    • Application Icon
    • Maximizing Only the Main Window
    • S6 Overlay Documentation

Images

Different docker images are available:

Base distribution	Tag	Size
Alpine 3.5	alpine-3.5
Alpine 3.6	alpine-3.6
Alpine 3.7	alpine-3.7
Alpine 3.8	alpine-3.8
Alpine 3.9	alpine-3.9
Alpine 3.10	alpine-3.10
Alpine 3.11	alpine-3.11
Alpine 3.12	alpine-3.12
Alpine 3.5	alpine-3.5-glibc
Alpine 3.6	alpine-3.6-glibc
Alpine 3.7	alpine-3.7-glibc
Alpine 3.8	alpine-3.8-glibc
Alpine 3.9	alpine-3.9-glibc
Alpine 3.10	alpine-3.10-glibc
Alpine 3.11	alpine-3.11-glibc
Alpine 3.12	alpine-3.12-glibc
Debian 8	debian-8
Debian 9	debian-9
Debian 10	debian-10
Ubuntu 16.04 LTS	ubuntu-16.04
Ubuntu 18.04 LTS	ubuntu-18.04

Due to its size, the Alpine image is recommended. However, it may be harder to integrate your application (especially third party ones without source code), because:

1. Packages repository may not be as complete as Ubuntu/Debian.
2. Third party applications may not support Alpine.
3. The Alpine distribution uses the musl C standard library instead of GNU C library (glibc).

Note that using the Alpine image with glibc integrated (alpine-3.5-glibc tag) may ease integration of applications.

The next choice is to use the Debian image. It provides a great compatibility and its size is smaller than the Ubuntu one. Finally, if for any reason you prefer an Ubuntu image, one based on the stable 16.04 LTS version is provided.

Content

Here are the main components of the baseimage:

• S6-overlay, a process supervisor for containers.
• x11vnc, a X11 VNC server.
• xvfb, a X virtual framebuffer display server.
• openbox, a windows manager.
• noVNC, a HTML5 VNC client.
• NGINX, a high-performance HTTP server.
• stunnel, a proxy encrypting arbitrary TCP connections with SSL/TLS.
• Useful tools to ease container building.
• Environment to better support dockerized applications.

Versioning

Images are versioned. Version number is in the form MAJOR.MINOR.PATCH, where an increment of the:

• MAJOR version indicates that a backwards-incompatible change has been done.
• MINOR version indicates that functionality has been added in a backwards-compatible manner.
• PATCH version indicates that a bug fix has been done in a backwards-compatible manner.

Tags

For each distribution-specific image, multiple tags are available:

Getting started

The Dockerfile for your application can be very simple, as only three things are required:

• Instructions to install the application.
• A script that starts the application (stored at /startapp.sh in container).
• The name of the application.

Here is an example of a docker file that would be used to run the xterm terminal.

In Dockerfile:

# Pull base image.
FROM jlesage/baseimage-gui:alpine-3.6

# Install xterm.
RUN add-pkg xterm

# Copy the start script.
COPY startapp.sh /startapp.sh

# Set the name of the application.
ENV APP_NAME="Xterm"

In startapp.sh:

#!/bin/sh
exec /usr/bin/xterm

Then, build your docker image:

docker build -t docker-xterm .

And run it:

docker run --rm -p 5800:5800 -p 5900:5900 docker-xterm

You should be able to access the xterm GUI by opening in a web browser:

http://[HOST IP ADDR]:5800

Environment Variables

Some environment variables can be set to customize the behavior of the container and its application. The following list give more details about them.

Environment variables can be set directly in your Dockerfile via the ENV instruction or dynamically by adding one or more arguments -e "<VAR>=<VALUE>" to the docker run command.

Config Directory

Inside the container, the application's configuration should be stored in the /config directory.

This directory is also used to store the VNC password. See the VNC Pasword section for more details.

NOTE: By default, during the container startup, the user which runs the application (i.e. user defined by USER_ID) will claim ownership of the entire content of this directory. This behavior can be changed via the TAKE_CONFIG_OWNERSHIP environment variable. See the Environment Variables section for more details.

Ports

Here is the list of ports used by container. They can be mapped to the host via the -p <HOST_PORT>:<CONTAINER_PORT> parameter. The port number inside the container cannot be changed, but you are free to use any port on the host side.

User/Group IDs

When using data volumes (-v flags), permissions issues can occur between the host and the container. For example, the user within the container may not exists on the host. This could prevent the host from properly accessing files and folders on the shared volume.

To avoid any problem, you can specify the user the application should run as.

This is done by passing the user ID and group ID to the container via the USER_ID and GROUP_ID environment variables.

To find the right IDs to use, issue the following command on the host, with the user owning the data volume on the host:

id <username>

Which gives an output like this one:

uid=1000(myuser) gid=1000(myuser) groups=1000(myuser),4(adm),24(cdrom),27(sudo),46(plugdev),113(lpadmin)

The value of uid (user ID) and gid (group ID) are the ones that you should be given the container.

Locales

The default locale of the container is set to POSIX. If this cause issues with your application, the proper locale can be set via your Dockerfile, by adding these two lines:

ENV LANG=en_US.UTF-8
RUN locale-gen en_CA.UTF-8

NOTE: Locales are not supported by musl C standard library on Alpine. See:

• http://wiki.musl-libc.org/wiki/Open_Issues#C_locale_conformance
• https://github.com/gliderlabs/docker-alpine/issues/144

Accessing the GUI

Assuming that container's ports are mapped to the same host's ports, the graphical interface of the application can be accessed via:

• A web browser:

http://<HOST IP ADDR>:5800

• Any VNC client:

<HOST IP ADDR>:5900

Security

By default, access to the application's GUI is done over an unencrypted connection (HTTP or VNC).

Secure connection can be enabled via the SECURE_CONNECTION environment variable. See the Environment Variables section for more details on how to set an environment variable.

When enabled, application's GUI is performed over an HTTPs connection when accessed with a browser. All HTTP accesses are automatically redirected to HTTPs.

When using a VNC client, the VNC connection is performed over SSL. Note that few VNC clients support this method. SSVNC is one of them.

SSVNC

SSVNC is a VNC viewer that adds encryption security to VNC connections.

While the Linux version of SSVNC works well, the Windows version has some issues. At the time of writing, the latest version 1.0.30 is not functional, as a connection fails with the following error:

ReadExact: Socket error while reading

However, for your convienence, an unoffical and working version is provided here:

https://github.com/jlesage/docker-baseimage-gui/raw/master/tools/ssvnc_windows_only-1.0.30-r1.zip

The only difference with the offical package is that the bundled version of stunnel has been upgraded to version 5.49, which fixes the connection problems.

Certificates

Here are the certificate files needed by the container. By default, when they are missing, self-signed certificates are generated and used. All files have PEM encoded, x509 certificates.

NOTE: To prevent any certificate validity warnings/errors from the browser or VNC client, make sure to supply your own valid certificates.

NOTE: Certificate files are monitored and relevant daemons are automatically restarted when changes are detected.

VNC Password

To restrict access to your application, a password can be specified. This can be done via two methods:

• By using the VNC_PASSWORD environment variable.
• By creating a .vncpass_clear file at the root of the /config volume. This file should contains the password in clear-text. During the container startup, content of the file is obfuscated and moved to .vncpass.

The level of security provided by the VNC password depends on two things:

• The type of communication channel (encrypted/unencrypted).
• How secure access to the host is.

When using a VNC password, it is highly desirable to enable the secure connection to prevent sending the password in clear over an unencrypted channel.

Access to the host by unexpected users with sufficient privileges can be dangerous as they can retrieve the password with the following methods:

• By looking at the VNC_PASSWORD environment variable value via the docker inspect command. By defaut, the docker command can be run only by the root user. However, it is possible to configure the system to allow the docker command to be run by any users part of a specific group.
• By decrypting the /config/.vncpass file. This requires the user to have the appropriate permission to read the file: it has to be root or be the user defined by the USER_ID environment variable. Also, to be able to retrieve the correct decryption key, one needs to know that the content of the file was generated by x11vnc.

DH Parameters

Diffie-Hellman (DH) parameters define how the DH key-exchange is performed. More details about this algorithm can be found on the OpenSSL Wiki.

DH Parameters are saved into the PEM encoded file located inside the container at /config/certs/dhparam.pem. By default, when this file is missing, 2048 bits DH parameters are automatically generated. Note that this one-time operation takes some time to perform and increases the startup time of the container.

Building A Container

This section provides useful tips for building containers based on this baseimage.

Selecting Baseimage Tag

Properly select the baseimage tag to use. For a better control and prevent breaking your container, use a tag for an exact version of the baseimage (e.g. alpine-3.6-v2.0.0). Using the latest version of the baseimage (alpine-3.6) is not recommended, since automatic upgrades between major versions will probably break your container build/execution.

Referencing Linux User/Group

Reference the Linux user/group under which the application is running by its ID (USER_ID/GROUP_ID) instead of its name. Name could change in different baseimage versions while the ID won't.

Default Configuration Files

Default configuration files should be stored in /defaults in the container.

Adding/Removing Packages

To add or remove packages, use the helpers add-pkg and del-pkg provided by this baseimage. To minimze the size of the container, these tools perform proper cleanup and make sure that no useless files are left after an addition or removal of packages.

Also, when packages need to be added temporarily, use the --virtual NAME parameter. This allows installing missing packages and then remove them easily using the provided NAME (no need to repeat given packages). Note that if a specified package is already installed, it will be ignored and will not be removed automatically.

Here is an example of a command that could be added to Dockerfile to compile a project:

RUN \
    add-pkg --virtual build-dependencies build-base cmake git && \
    # Compile your project here...
    git clone https://myproject.com/myproject.git
    ... && \
    del-pkg build-dependencies

Supposing that, in the example above, the git package is already installed when the call to add-pk is performed, running del-pkg build-dependencies doesn't remove it.

Modifying Files With Sed

sed is a useful tool and is often used in container builds to modify files. However, one downside of this method is that there is no easy way to determine if sed actually modified the file or not.

It's for this reason that the baseimage includes a helper that gives sed a "patch-like" behavior: if the application of a sed expression results in no change on the target file, then an error is reported. This helper is named sed-patch and has the following usage:

sed-patch [SED_OPT]... SED_EXPRESSION FILE

Note that the sed option -i (edit files in place) is already supplied by the helper.

It can be used in Dockerfile, for example, like this:

RUN sed-patch 's/Replace this/By this/' /etc/myfile

If running this sed expression doesn't bring any change to /etc/myfiles, the command fails and thus, the Docker build also.

Modifying Baseimage Content

Try to minimize modifications to files provided by the baseimage. This minimizes to risk of breaking your container after using a new baseimage version.

Application's Data

Applications often needs to write configuration, data, logs, etc. Always make sure they are all written under /config. This directory is a volume intended to be mapped to a folder on the host. The goal is to write stuff outside the container and keep these data persistent.

A lot of applications use the environment variables defined in the XDG Base Directory Specification to determine where to store various data. The baseimage sets these variables so they all fall under /config/:

• XDG_DATA_HOME=/config/xdg/data
• XDG_CONFIG_HOME=/config/xdg/config
• XDG_CACHE_HOME=/config/xdg/cache

$HOME Environment Variable

The application is run under a user having its own UID. This user can't be used to login with, has no password, no valid login shell and no home directory. It is effectively a kind of user used by daemons.

Thus, by default, the $HOME environment variable is not set. While this should be fine in most case, some applications may expect the $HOME environment variable to be set (since normally the application is run by a logged user) and may not behave correctly otherwise.

To make the application happy, the home directory can be set at the beginning of the startapp.sh script:

export HOME=/config

Adjust the location of the home directory to fit your needs. However, if the application uses the home directory to write stuff, make sure it is done in a volume mapped to the host (e.g. /config),

Note that the same technique can be used by services, by exporting the home directory into their run script.

Service Dependencies

When running multiple services, service srvB may need to start only after service SrvA.

Service dependencies are defined by creating a regular file in the service's directory, its name being the name of the dependent service with the .dep extension. For example, touching the file:

/etc/services.d/srvB/srvA.dep

indicates that service srvB depends on service srvA.

Service Readiness

By default, a service is considered ready when the supervisor successfully forked and executed the daemon. However, some daemons do a lot of initialization work before they're actually ready to serve.

Hopefully, the S6 supervisor supports service startup notifications. This is a simple mechanism allowing daemons to notify the supervisor when they are ready to serve.

While support for this mechanism can be implemented natively in the daemon, the use of the s6-notifyoncheck program makes it possible for services to use the S6 notification mechanism with any daemon.

Log Monitor

This baseimage include a simple log monitor. This monitor allows sending notification(s) when a particular message is detected in a log file.

This system has two main component: notification definitions and notifications backends (targets). Definitions describe properties of a notification (title, message, severity, etc) and how it is triggered (i.e. filtering function). Once a matching string is found in a log file, a notification is triggered and sent via one or more backends. A backend can implement any functionality. For example, it could send the notification to the standard output, a file or an online service.

Monitored Files

File(s) to be monitored can be set in the configuration file located at /etc/logmonitor/logmonitor.conf. There are two settings to look at:

• LOG_FILES: List of absolute paths to log files to be monitored. A log file is a file having new content appended to it.
• STATUS_FILES: List of absolute paths to status files to be monitored. A status file doesn't have new content appended. Instead, its whole content is refreshed/overwritten periodically.

Notification Definition

The definition of a notification consists in multiple files, stored in a directory under /etc/logmonitor/notifications.d. For example, definition of notification NOTIF is found under /etc/logmonitor/notifications.d/NOTIF/. The following table describe files part of the definition:

Notification Backend

Definition of notification backend is stored in a directory under /etc/logmonitor/targets.d. For example, definition of STDOUT backend is found under /etc/logmonitor/notifications.d/STDOUT/. The following table describe files part of the definition:

By default, the baseimage contains the following notification backends:

Application Icon

A picture of your application can be added to the image. This picture is displayed in the WEB interface's navigation bar. Also, multiple favicons are generated, supporting all browsers and platforms.

Add the following command to your Dockerfile, with the proper URL pointing to your master icon: The master icon should be a square PNG image with a size of at least 260x260 for optimal results.

# Generate and install favicons.
RUN \
    APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
    install_app_icon.sh "$APP_ICON_URL"

Favicons are generated by RealFaviconGenerator. You can tweak yourself their display with the following method:

• Generate favicons yourself with RealFaviconGenerator.
  • Set the path to /images/icons/.
  • Enable versioning and set it to v=ICON_VERSION.
• At the installation page, choose the Node CLI tab.
• Copy the content of faviconDescription.json.
• Minify the JSON using an online JSON minifier.
  • Before running the minifier, modify the masterPicture field to /opt/novnc/images/icons/master_icon.png.
• Copy-paste the result in your Dockerfile. It will be passed to the install script.
• Your Dockerfile should have something like:

# Generate and install favicons.
RUN \
    APP_ICON_URL=https://github.com/jlesage/docker-templates/raw/master/jlesage/images/generic-app-icon.png && \
    APP_ICON_DESC='{"masterPicture":"/opt/novnc/images/icons/master_icon.png","iconsPath":"/images/icons/","design":{"ios":{"pictureAspect":"backgroundAndMargin","backgroundColor":"#ffffff","margin":"14%","assets":{"ios6AndPriorIcons":false,"ios7AndLaterIcons":false,"precomposedIcons":false,"declareOnlyDefaultIcon":true}},"desktopBrowser":{},"windows":{"pictureAspect":"noChange","backgroundColor":"#2d89ef","onConflict":"override","assets":{"windows80Ie10Tile":false,"windows10Ie11EdgeTiles":{"small":false,"medium":true,"big":false,"rectangle":false}}},"androidChrome":{"pictureAspect":"noChange","themeColor":"#ffffff","manifest":{"display":"standalone","orientation":"notSet","onConflict":"override","declared":true},"assets":{"legacyIcon":false,"lowResolutionIcons":false}},"safariPinnedTab":{"pictureAspect":"silhouette","themeColor":"#5bbad5"}},"settings":{"scalingAlgorithm":"Mitchell","errorOnImageTooSmall":false},"versioning":{"paramName":"v","paramValue":"ICON_VERSION"}}' && \
    install_app_icon.sh "$APP_ICON_URL" "$APP_ICON_DESC"

Maximizing Only the Main Window

By default, the application's window is maximized and decorations are hidden. However, when the application has multiple windows, this behavior may need to be restricted only to the main one.

This can be achieved by matching on more window parameters: class, name, role, title and type. By default, only the type parameter is used and must equal to normal.

To find all parameters of the main window:

• While the application is running and the main window is focused, login to the container.

docker exec -ti [container name or id] sh

• Execute obxprop --root | grep "^_NET_ACTIVE_WINDOW". The output will look like:

_NET_ACTIVE_WINDOW(WINDOW) = 16777220

• Using this ID, show the parameters by executing obxprop --id [MAIN WINDOW ID] | grep "^_OB_APP". The output will look like:

_OB_APP_TYPE(UTF8_STRING) = "normal"
_OB_APP_CLASS(UTF8_STRING) = "Google-chrome"
_OB_APP_NAME(UTF8_STRING) = "google-chrome"
_OB_APP_ROLE(UTF8_STRING) =
_OB_APP_TITLE(UTF8_STRING) = "Google Chrome"

Finally, in the Dockerfile of your container, modify the configuration file of openbox (located at /etc/xdg/openbox/rc.xml) to apply window restriction. Usually, specifying the window's title is enough.

sed-patch 's/<application type="normal">/<application type="normal" title="Google Chrome">/' /etc/xdg/openbox/rc.xml

See the openbox's documentation for more details: http://openbox.org/wiki/Help:Applications

S6 Overlay Documentation

• Make sure to read the S6 overlay documentation. It contains information that can help building your image. For example, the S6 overlay allows you to easily add initialization scripts and services.