add postgres db (#26)

This PR moves the db service into two override files `compose.mariadb.yml` and `compose.postgres.yml`.

Existing installations need to add:
```
COMPOSE_FILE="compose.yml"
COMPOSE_FILE="$COMPOSE_FILE:compose.mariadb.yml"
```
to their .env file to ensure mariadb is still used.

Co-authored-by: Philipp Rothmann <philipprothmann@posteo.de>
Reviewed-on: #26

.env.sample

@@ -5,6 +5,10 @@ DOMAIN=nextcloud.example.com
 #EXTRA_DOMAINS=', `www.nextcloud.example.com`'
 LETS_ENCRYPT_ENV=production
 
+COMPOSE_FILE="compose.yml"
+COMPOSE_FILE="$COMPOSE_FILE:compose.mariadb.yml"
+#COMPOSE_FILE="$COMPOSE_FILE:compose.postgres.yml"
+
 ADMIN_USER=admin
 
 SECRET_DB_ROOT_PASSWORD_VERSION=v1