Move python package installation to a docker image

[ch-iv]

When rails is started using docker, entrypoint-dev-rails.ssh tries to install all python packages even if they have already been installed. This significantly slows down the time it takes for rails to start up in dev.

This could be fixed by moving the step of installing python packages to a docker image. Then packages won't be reinstalled every single time, because the docker image would be cached.

The same could probably also be done to node packages and bundle gems.