Fix Dockerfile not using current sources

[lukel97]

The Dockerfile previously cloned the LNT sources as a RUN step, which meant that when rebuilding the image any changes to the LNT sources wouldn't be picked up because it would be cached.

This PR fixes it alongside some other things:

• Don't clone the sources in a RUN step, instead COPY them from the build context
• Move the Dockerfile/docker-compose.yml files to the root so the source files are in the build directory
• Split out the install step so that we only COPY over the files necessary to run the install (to aid with layer caching)
• In the docker-compose.yml file, postgres:11 doesn't seem to exist anymore on docker.hub so upgrade it to 13, and fully qualify it so it can be used by podman
• Remove the .env file and just write the defaults directly in the docker-compose.yml file

This also downgrades Jinja2 and pyyaml again to get building with Python 3.10, but these can probably be fixed in another PR