Remove uid/gid build arguments to FlowDB

[greenape]

FlowDB currently takes two build arguments POSTGRES_UID and POSTGRES_GID which were necessary to enable one to work around conflicts between the user Postgres ran as, and the uids on the host system. Per
docker-library/postgres#253 and
docker-library/postgres#448 the upstream image now supports setting these via --user to match an existing user on the host system. Using this method also means docker won't clobber permissions when spinning up the Postgres instance if you don't have a user matching the internal uid & gid.

We should remove the old workaround, and explain how to do this in the docs.