Implement minor build fixes

This PR introduces a number of relatively minor container building and configuration related fixes that I've been collecting while developing.

Rather than randomly creating a new django secret key every time the django container is built, we set the secret key using the .env file. This allows us to keep the same secret key across multiple container builds and also make use of django settings for linting and type checking in development.
There were two "versions" of the same version of solrpy running around. The one on pypi had a very annoying extra print statement that clogged up the logs, while the release on the solrpy repository did not have that print statement. This PR switches those out in the python dependencies.
This PR adds the psycopg binary version to our dependencies, since we don't need to use the pure python implementation.
In #819, we switched from using the PORT environment variable to signal when we were running in development mode to using a more aptly named DEVELOPMENT variable, but we didn't modify the rest of the app container build process to properly make use of this new variable. We do that here.
The celery app was using some convoluted methods to get its settings. We simplify that in 9a21998

DDMAL / cantus