This change adds a dependency to the (nebraska) "server" service on the
"postgres" service in docker-compose.test.yaml.
Motivation of the change is to prevent a race condition where the
"server" container starts faster than "postgres", fails to connect to
the database, and shuts down.

Additionally, an optional persistent storage for postgres is added.
DB contents can be persisted across docker-compose runs to aid manual,
interactive testing. Usage instructions have been added to the YAML
files as comments. The local directory used for persiting postgres data
has been added to .gitignore to prevent accidental commits.

Signed-off-by: Thilo Fromm <thilofromm@microsoft.com>

…ice restart

Co-authored-by: Kai Lüke <pothos@users.noreply.github.com>