Fix "make install" error on pristine clone

[camilstaps]

This is my first PR, so I apologize if I'm doing something wrong here.

• The change to docker-compose.yml gives postgres a different data directory from the mount path, which is non-empty (.gitignore). This is what is suggested in https://serverfault.com/questions/1018377/postgres-mount-volume-error-in-k8s. Closes .gitignore in db/postgres-data prevents initdb from initializing postgres cluster in docker service janeway-postgres #2775.
• The change to Makefile makes sure that after deleting db/* the contents of this directory is reinitialized from the git index; otherwise you are left with a changeset that you don't want to commit. This is not really related to the previous point, but I thought it was too small to make a separate issue and PR. If you prefer I can remove this commit of course.

[Bbkctp]

Can one of the admins verify this patch?