A minimal Ubuntu base image that is Predictable and stays Up To Date
ubuntu-baseimage is a docker base image based on the work of phusion/baseimage-docker and inherits the following features:
- A correct Init system solving the PID 1 problem.
- Scripts in
/etc/my_init.d/are started in lexical order. rc.localis executed after the Init Scripts.- Fixes APT incompatibilities with Docker See moby/moby#1024.
- A mechansim to simplify work with Environment Variables.
- Provides a
cronandsyslog-ngwithlogotatedaemon per default. - Ability to define additonal daemons in
/etc/service. - Lightweight sudo alternative at
/sbin/setuser
Moreover ubuntu-baseimage brings the following improvements:
- Provides an Automated Build that stays up to date and
eleminates the need to run
apt-get upgrade. (See dockerfile best-practices) - Services are started in lexical order.
- Services are terminated in reverse lexical order.
- User command is executed after all services has been started.
- If a
checkscript for a service is provided, the init daemon will wait until service is fully started. /sbin/my_initinDockerfileis defined asENTRYPOINTinstead asCMD.
Changes due to personal choice:
- Removes sshd service as it is not essential (see nsenter).
ubuntu-baseimage introduces backward-incompatible changes to phusion/baseimage-docker.
The problems ubuntu-baseimage tries to solve
Even though phusion/baseimage-docker claims that it does everything right, it still leaves us with some unsolved problems:
- The image is not updated regularly (Last push was 2 months ago, whereas Ubuntu base Image was pushed 8 days ago. Status from 10th Sep 2015). Moreover it advocates the use of
apt-get upgradewhich is contrary to dockerfile best-practices) - Naturally services/daemons depend on each other. phusion/baseimage-docker starts and stops all daemons at the same time causing unpredictable behavior and failure.
- phusion/baseimage-docker executes the user provided command without knowing if the required services/daemons are up and running.
ubuntu-baseimage solves the above problems using an Automated Build and a predictable boot and shutdown mechanism.
Using ubuntu-baseimage as base image
- Getting started
- Adding additional daemons
- Disable daemons
- Running scripts during container startup
- Environment variables
- Upgrading the operating system inside the container
Getting started
The image is called wingedkiwi/ubuntu-baseimage, and is available on the Docker registry.
# Use wingedkiwi/ubuntu-baseimage as base image.
FROM wingedkiwi/ubuntu-baseimage:<VERSION>
# Put your own build instructions here.
# Clean up APT when done.
RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
# Run your command or leave empty if only the daemons should be started.
CMD ["your_command"]
Adding additional daemons
You can add additional daemons (e.g. your own app) to the image by creating runit entries. You only have to write a small shell script which runs your daemon, and runit will keep it up and running for you, restarting it when it crashes, etc.
The shell script must be called run, must be executable, and is to be placed in the directory /etc/service/<NAME>.
Here's an example showing you how a memcached server runit entry can be made.
In memcached.sh (make sure this file is chmod +x):
#!/bin/sh
# `/sbin/setuser memcache` runs the given command as the user `memcache`.
# If you omit that part, the command will be run as root.
exec /sbin/setuser memcache /usr/bin/memcached >>/var/log/memcached.log 2>&1
In Dockerfile:
RUN mkdir /etc/service/30-memcached
ADD memcached.sh /etc/service/30-memcached/run
Note that the shell script must run the daemon without letting it daemonize/fork it. Usually, daemons provide a command line flag or a config file option for that.
Starting order
Daemons are started in lexical order. System services are defined as 10-syslog-ng, 10-syslog-forwarder and 20-cron, so you can define your daemons e.g. starting with 30 to start your daemon after the system services.
Wait until daemon is fully up
You can add a check script to make the Init System wait until daemon is fully up.
Example memcached-check.sh file:
#!/bin/bash
if [ $memcached_is_up = true ]; then
exit 0
fi
exit 1
And in Dockerfile:
ADD memcached-check.sh /etc/service/30-memcached/check
Disable Daemons
Any daemon can be disabled by adding a disabled file to the corresponding service folder.
RUN touch /etc/service/30-memcached/disabled
Running scripts during container startup
The ubuntu-baseimage init system, /sbin/my_init, runs the following scripts during startup, in the following order:
- All executable scripts in
/etc/my_init.d, if this directory exists. The scripts are run in lexicographic order. - The script
/etc/rc.local, if this file exists.
All scripts must exit correctly, e.g. with exit code 0. If any script exits with a non-zero exit code, the booting will fail.
The following example shows how you can add a startup script. This script simply logs the time of boot to the file /tmp/boottime.txt.
In logtime.sh (make sure this file is chmod +x):
#!/bin/sh
date > /tmp/boottime.txt
In Dockerfile:
RUN mkdir -p /etc/my_init.d
ADD logtime.sh /etc/my_init.d/logtime.sh
Environment variables
If you use /sbin/my_init as the main container command, then any environment variables set with docker run --env or with the ENV command in the Dockerfile, will be picked up by my_init. These variables will also be passed to all child processes, including /etc/my_init.d startup scripts, Runit and Runit-managed services. There are however a few caveats you should be aware of:
- Environment variables on Unix are inherited on a per-process basis. This means that it is generally not possible for a child process to change the environment variables of other processes.
- Because of the aforementioned point, there is no good central place for defining environment variables for all applications and services. Debian has the
/etc/environmentfile but it only works in some situations. - Some services change environment variables for child processes. Nginx is one such example: it removes all environment variables unless you explicitly instruct it to retain them through the
envconfiguration option. If you host any applications on Nginx then they will not see the environment variables that were originally passed by Docker.
my_init provides a solution for all these caveats.
Centrally defining your own environment variables
During startup, before running any startup scripts, my_init imports environment variables from the directory /etc/container_environment. This directory contains files who are named after the environment variable names. The file contents contain the environment variable values. This directory is therefore a good place to centrally define your own environment variables, which will be inherited by all startup scripts and Runit services.
For example, here's how you can define an environment variable from your Dockerfile:
RUN echo Apachai Hopachai > /etc/container_environment/MY_NAME
You can verify that it works, as follows:
$ docker run -t -i <YOUR_NAME_IMAGE> /sbin/my_init -- bash -l
...
*** Running bash -l...
# echo $MY_NAME
Apachai Hopachai
Handling newlines
If you've looked carefully, you'll notice that the 'echo' command actually prints a newline. Why does $MY_NAME not contain a newline then? It's because my_init strips the trailing newline, if any. If you intended on the value having a newline, you should add another newline, like this:
RUN echo -e "Apachai Hopachai\n" > /etc/container_environment/MY_NAME
Environment variable dumps
While the previously mentioned mechanism is good for centrally defining environment variables, it by itself does not prevent services (e.g. Nginx) from changing and resetting environment variables from child processes. However, the my_init mechanism does make it easy for you to query what the original environment variables are.
During startup, right after importing environment variables from /etc/container_environment, my_init will dump all its environment variables (that is, all variables imported from container_environment, as well as all variables it picked up from docker run --env) to the following locations, in the following formats:
/etc/container_environment/etc/container_environment.sh- a dump of the environment variables in Bash format. You can source the file directly from a Bash shell script./etc/container_environment.json- a dump of the environment variables in JSON format.
The multiple formats makes it easy for you to query the original environment variables no matter which language your scripts/apps are written in.
Here is an example shell session showing you how the dumps look like:
$ docker run -t -i \
--env FOO=bar --env HELLO='my beautiful world' \
wingedkiwi/ubuntu-baseimage:<VERSION> bash -l
...
*** Running bash -l...
# ls /etc/container_environment
FOO HELLO HOME HOSTNAME PATH TERM container
# cat /etc/container_environment/HELLO; echo
my beautiful world
# cat /etc/container_environment.json; echo
{"TERM": "xterm", "container": "lxc", "HOSTNAME": "f45449f06950", "HOME": "/root", "PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "FOO": "bar", "HELLO": "my beautiful world"}
# source /etc/container_environment.sh
# echo $HELLO
my beautiful world
Modifying environment variables
It is even possible to modify the environment variables in my_init (and therefore the environment variables in all child processes that are spawned after that point in time), by altering the files in /etc/container_environment. After each time my_init runs a startup script, it resets its own environment variables to the state in /etc/container_environment, and re-dumps the new environment variables to container_environment.sh and container_environment.json.
But note that:
- modifying
container_environment.shandcontainer_environment.jsonhas no effect. - Runit services cannot modify the environment like that.
my_initonly activates changes in/etc/container_environmentwhen running startup scripts.
Security
Because environment variables can potentially contain sensitive information, /etc/container_environment and its Bash and JSON dumps are by default owned by root, and accessible only by the docker_env group (so that any user added this group will have these variables automatically loaded).
If you are sure that your environment variables don't contain sensitive data, then you can also relax the permissions on that directory and those files by making them world-readable:
RUN chmod 755 /etc/container_environment
RUN chmod 644 /etc/container_environment.sh /etc/container_environment.json
Upgrading the operating system inside the container
Upgrading inside the container would violate docker best practices. ubuntu-baseimage is always kept up to date using an Automated Build on Docker Hub. Simply rebuild your container using this base image.