Remove google_auth_client_secret from docker compose

[MelchiorKokernoot]

https://github.com/eyra/mono/blob/master/docker-compose.yml#L28

Shows the google_auth_client_secret, although this secret is (at a glance) not used anywhere, this secret should not be publicly visible and should be deleted from the current version.

Scope
For now, it is not required to remove the secret from the entire VCS history, Melle implied that more of these "should permanently deleted" parts exist. These should be identified and permanently deleted in batch, which will follow from a new issue