Support for env vars-driven Postgresql connection setup

The setup provided by the docker image is minimalistic - it only:

• assures default values of PG_HOST (127.0.0.1), PG_PORT (5432), PG_USER (www-data) and PG_DBNAME (www-data)
• exports PG_CONN="-h $PG_HOST -p $PG_PORT -U $PG_USER $PG_DBNAME" (to be used as psql $PG_CONN)
• sets PG_EXTERNAL=1 if PG_HOST is provided

The actual use of these env vars must be implemented in the configuration repository.