Modify docker-compose.yml to reduce running/exited services / persistent volume

[mbeacom]

The current docker-compose.yml defines shell and migration service containers.
I am uncertain what function the shell container provides (if you wish to access the shell of the running web container, you can docker exec -it against it).

Currently, the postgresql data lives as long as the underlying container.
We can preserve the data via a persistent volume.

Additionally, the migration service will always end as exited, meanwhile the docker-compose.bash script runs the migration step before running the test server.

I am proposing we replace the migration service container with either a shell script, makefile entry, README documentation, or a combination to allow users to easily make migrations and migrate the database in the event it's necessary.

The proposed changes will eliminate two of four containers.