Build a Dockerfile

In this lab you will begin working on the Dockerfile that will be the heart of your project. The Dockerfile packages your application to be run as a container. The instructions will start you off with a Dockerfile that works for your project if you completed the A Django Application milestone.

Starter Dockerfile

In the root directory in your repo create a new Dockerfile. The initial contents of your Dockerfile should be:

FROM docker.io/python:3.10

RUN pip install django 
COPY mysite /mysite 
WORKDIR /mysite 
CMD python ./manage.py runserver 0.0.0.0:8000

The initial Dockerfile expects that the mysite directory exists in the root of your repo. Note that it runs the pip install django command that we ran before the Build a Django Application lab. If that command is not run Django won’t be installed in the container.

Keep Up With Changes

In future labs you will use the pip and apt commands to install new software. It’s your responsibility to make sure those commands are also run in your Dockerfile so that your container is updated.

Build and run your container using the instructions in the Build a Docker Container lab. Make sure you start the container with a port definition like this:

$ docker run -it --rm -p 8000:8000 mysite 

Debugging a Container That Won’t Start

Sometimes a container builds but won’t start. Problems like this arise when there are problems with the WORKDIR or CMD lines in the Dockerfile or when a COPY moved files to the wrong place. When a container won’t start it can be hard to figure out what went wrong. When you run a container you can override the CMD in the Dockerfile to get a shell. The shell lets you look around and try to see what’s wrong.

$ docker run -it --rm -p 8080:8000 mysite /bin/bash

With bash running you can look around the container with cd and ls and also try to start your application manually:

root@9e30cbcaa3af:/mysite# python3 ./manage.py runserver 0.0.0.0:8000 
Watching for file changes with StatReloader
Performing system checks...

System check identified no issues (0 silenced).
February 15, 2023 - 01:14:12
Django version 4.1.7, using settings 'mysite.settings'
Starting development server at http://0.0.0.0:8000/
Quit the server with CONTROL-C.