Adds in environment variable in gtm environment for PG data dir (#228)

@see docker-library/postgres#1259

Author: typhonius

.data/gtm/compose.yaml

@@ -12,6 +12,7 @@ services:
       PGUSER: postgres
       POSTGRES_USER: postgres
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      PGDATA: /var/lib/postgresql/data
     ports:
       - "5432:5432"
     healthcheck: