Docker: Configure keycloak to use the postgresql (or MSSQL) db for persistence

[gingters]

In order to show how to properly set up a keycloak, we shouldn't use in-container file based storage, but store the keycloak data in a database.
Since we already have a Postgres / SQL Server db, we can use that by simply changing the keycloak config. Keycloak uses its own db schema, so that should not conflict with our relayserver db.