Why does CMD never work in my Dockerfiles? - django

Question

I have a few Dockerfiles where CMD doesn't seem to run. Here is an example (all the way at the bottom).
##########################################################
# Set the base image to Ansible
FROM ubuntu:16.10
# Install Ansible, Python and Related Deps #
RUN apt-get -y update && \
apt-get install -y python-yaml python-jinja2 python-httplib2 python-keyczar python-paramiko python-setuptools python-pkg-resources git python-pip
RUN mkdir /etc/ansible/
RUN echo '[local]\nlocalhost\n' > /etc/ansible/hosts
RUN mkdir /opt/ansible/
RUN git clone http://github.com/ansible/ansible.git /opt/ansible/ansible
WORKDIR /opt/ansible/ansible
RUN git submodule update --init
ENV PATH /opt/ansible/ansible/bin:/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin
ENV PYTHONPATH /opt/ansible/ansible/lib
ENV ANSIBLE_LIBRARY /opt/ansible/ansible/library
RUN apt-get update -y
RUN apt-get install python -y
RUN apt-get install python-dev -y
RUN apt-get install python-setuptools -y
RUN apt-get install python-pip
RUN mkdir /ansible/
WORKDIR /ansible
COPY ./ansible ./
WORKDIR /
RUN ansible-playbook -c local ansible/playbooks/installdjango.yml
ENV PROJECTNAME testwebsite
################## SETUP DIRECTORY STRUCTURE ######################
WORKDIR /home
CMD ["django-admin" "startproject" "$PROJECTNAME"]
EXPOSE 8000
If I build and run the container, I can manually run
Django-admin startproject $PROJECTNAME and it will create a new project as expected, but the CMD in my Dockerfile does not seem to be doing anything and this is happening with all my other Dockerfiles so there's something I must not be getting.

Answer 1

ENTRYPOINT and CMD defines the default command that docker runs when it starts your container, not when the image is built. When ENTRYPOINT isn't defined, you simply run the value of CMD. Otherwise, CMD becomes args to the ENTRYPOINT. When you run your image, you can override the value of the CMD by passing args after the container name.
So, in your example above, CMD may be defined as anything, but when you run your container with docker run -it <imagename> /bin/bash, you override any value of CMD and replace it with /bin/bash. To run the defined value of CMD, you would need to run the container with docker run <imagename>.