This introduces new env variables to configure nextloud to be embedded via
iframe on an external site.
Setting X_FRAME_OPTIONS_ENABLED=1 will configure nginx and nextcloud to
set X-Frame-Options and CSP headers to allow the domain configured in
X_FRAME_OPTIONS_ALLOW_FROM.
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: coop-cloud/nextcloud#26
