Typically, when we build a container image we have 2 main files:

Dockerfile is the essence of the container, it is what the container is made of, it generally contains packages installation steps and files
entrypoint.sh is where we configure the container, during the bootstrap sequence this script will get executed. Usually the entrypoint.sh file contains bash instructions.

So the idea is, instead of relying on bash scripting when writing container’s entrypoint we could call an Ansible to configure it.

Do not forget to replace all the my-application statement with the name of your application ;).

File example for base image Dockerfile that your application will be using:. We simple install Ansible and our application here:

FROM ubuntu:14.04
MAINTAINER Sébastien Han "seb@redhat.com"

# Install prerequisites
RUN apt-get update && \
    apt-get install -y python python-dev python-pip python-yaml && \
    apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

# Install Ansible
RUN pip install pyyaml ansible

# Install my application
RUN apt-get install -y --force-yes my-application && \
apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

RUN mkdir -p /opt/ansible/my-application/
ADD site.yml /opt/ansible/my-application/site.yml

ADD entrypoint.sh /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

File example for a site.yml, this file will later be used by Ansible in your application container:

---
# Defines deployment design and assigns role to server groups

- hosts: my-application
  connection: local
  sudo: True
  roles:
  - { role: application1, tags: installation }
  - { role: application2, tags: configuration }

File example for an application entrypoint.sh. This is some of the brief instructions that you will find to run Ansible:

cat >/opt/ansible/my-application/inventory <<EOF
[my-application]
127.0.0.1
EOF

cat >/opt/ansible/my-application/group_vars/all <<EOF
foo: bar
foo1: bar1
EOF

cd /opt/ansible/my-application
ansible-playbook -vvv -i inventory site.yml

Now simply run docker run <image> and your container will get configured by Ansible :). As always use docker logs -f <container-id> to check the bootstrap process.

Ansible power! Now it would be interesting to do a bit of profiling as Ansible might slow things down a little bit. From some of the test I ran, this is not much but it is up to you to decide whether it is acceptable or not.