Update Dockerfile to match Python template

[mythmon]

When I wrote the Dockerfile, I made up all the Python stuff because I didn't know there was a Python template I could reference. Now I know that python/Dockerfile.t exists.

The Dockerfile we use should be updated to work similarly to the Python template, especially making it use the Python buildpack.

We might run into some errors or false-errors around collectstatic. The reason we are using Docker in the first place is because the python buildpack does strange things that don't play nice with Node during build time.

[mythmon]

I figured out how to get the buildpacks to play nice, so we aren't using docker anymore.

[willkg]

Hooray! Congratulations!