dragonmantank / baseimage-docker

A minimal Docker base image with a correct and usable system

A minimal Docker base image with a correct and usable system

Baseimage-docker is a Docker image meant to serve as a good base for any other Docker container. It contains a minimal base system with the most important things already installed and set up correctly.

• Github: https://github.com/phusion/baseimage-docker
• Docker registry entry: https://index.docker.io/u/phusion/baseimage/
• Discussion forum: https://groups.google.com/d/forum/passenger-docker
• Twitter: https://twitter.com/phusion_nl
• Blog: http://blog.phusion.nl/

Why use baseimage-docker?

Why use baseimage-docker instead of doing everything yourself in Dockerfile?

• It reduces the time needed to write a correct Dockerfile. You won't have to worry about the base system and can focus on your stack and your app.
• It sets up the base system correctly. It's very easy to get the base system wrong, but this image does everything correctly.
• It reduces the time needed to run docker build, allowing you to iterate your Dockerfile more quickly.

Contents

Looking for a more complete base image, one that is ideal for Ruby, Python, Node.js and Meteor web apps? Take a look at passenger-docker.

Component	Why is it included? / Remarks
Ubuntu 12.04 LTS	The base system.
A correct init process	According to the Unix process model, the init process -- PID 1 -- inherits all orphaned child processes and must [reap them](https://en.wikipedia.org/wiki/Wait_(system_call). Most Docker containers do not have an init process that does this correctly, and as a result their containers become filled with zombie processes over time. Baseimage-docker comes with an init process /sbin/my_init that performs reaping correctly.
Fixes APT incompatibilities with Docker	See moby/moby#1024.
syslog-ng	A syslog daemon is necessary so that many services - including the kernel itself - can correctly log to /var/log/syslog. If no syslog daemon is running, a lot of important messages are silently swallowed. Only listens locally.
ssh server	Allows you to easily login to your container to inspect or administer things. Password and challenge-response authentication are disabled by default. Only key authentication is allowed. It allows an predefined key by default to make debugging easy. You should replace this ASAP. See instructions.
cron	The cron daemon must be running for cron jobs to work.
runit	For service supervision and management. Much easier to use than SysV init and supports restarting daemons when they crash. Much easier to use and more lightweight than Upstart.
setuser	A tool for running a command as another user. Easier to use than su, has a smaller attack vector than sudo, and unlike chpst this tool sets $HOME correctly. Available as /sbin/setuser.

Baseimage-docker is very lightweight: it only consumes 4 MB of memory.

Wait, I thought Docker is about running a single process in a container?

Absolutely not true. Docker runs fine with multiple processes in a container. In fact, there is no technical reason why you should limit yourself to one process - it only makes things harder for you and breaks all kinds of essential system functionality, e.g. syslog.

Baseimage-docker encourages multiple processes through the use of runit.

Inspecting baseimage-docker

To look around in the image, run:

docker run -rm -t -i phusion/baseimage bash -l

You don't have to download anything manually. The above command will automatically pull the baseimage-docker image from the Docker registry.

Using baseimage-docker as base image

The image is called phusion/baseimage, and is available on the Docker registry.

By default, it allows SSH access for the key in image/insecure_key. This makes it easy for you to login to the container, but you should replace this key as soon as possible.

# Use phusion/baseimage as base image. To make your builds reproducible, make
# sure you lock down to a specific version, not to `latest`!
# See https://github.com/phusion/baseimage-docker/blob/master/Changelog.md for
# a list of version numbers.
FROM phusion/baseimage:<VERSION>

# Set correct environment variables.
ENV HOME /root

# Remove authentication rights for insecure_key.
RUN rm -f /root/.ssh/authorized_keys /home/*/.ssh/authorized_keys

# Use baseimage-docker's init process.
CMD ["/sbin/my_init"]

# ...put other build instructions here...

# Clean up APT when done.
RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

Adding additional daemons

You can add additional daemons 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/services/<NAME>.

Here's an example showing you how to a memached server runit entry can be made.

### In memcached.sh (make sure this file is chmod +x):
#!/bin/sh
# `chpst` is part of running. `chpst -u memcache` runs the given command
# as the user `memcache`. If you omit this, the command will be run as root.
exec chpst -u memcache /usr/bin/memcached >>/var/log/memcached.log 2>&1

### In Dockerfile:
RUN mkdir /etc/services/memcached
ADD redis.sh /etc/services/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.

Login in to the container

You can use SSH to login to any container that is based on baseimage-docker.

Start a container based on baseimage-docker (or a container based on an image based on baseimage-docker):

docker run phusion/baseimage

Find out the ID of the container that you just ran:

docker ps

Once you have the ID, look for its IP address with:

docker inspect <ID> | grep IPAddress

Now SSH into the container. In this example we're using the default insecure key, but if you're followed the instructions well then you've already replaced that with your own key. You did replace the key, didn't you?

ssh -i insecure_key root@<IP address>

Building the image yourself

If for whatever reason you want to build the image yourself instead of downloading it from the Docker registry, follow these instructions.

Clone this repository:

git clone https://github.com/phusion/baseimage-docker.git
cd baseimage-docker

Start a virtual machine with Docker in it. You can use the Vagrantfile that we've already provided.

vagrant up
vagrant ssh
cd /vagrant

Build the image:

make build

If you want to call the resulting image something else, pass the NAME variable, like this:

make build NAME=joe/baseimage

Conclusion

• Using baseimage-docker? Tweet about us or follow us on Twitter.
• Having problems? Please post a message at the discussion forum.
• Looking for a more complete base image, one that is ideal for Ruby, Python, Node.js and Meteor web apps? Take a look at passenger-docker.

Please enjoy baseimage-docker, a product by Phusion. :-)