Use environment variables for the config.
See https://github.com/nextcloud/server/pull/3966/files#diff-604e02ade0fc8de03f58c2d78fc5aef3677de5e02f388df733c00e24dae01b29R43.
See https://github.com/nextcloud/server/pull/3966/files#diff-604e02ade0fc8de03f58c2d78fc5aef3677de5e02f388df733c00e24dae01b29R43.