Bugfix: Fix Docker configuration for `DISABLE_AUTHENTICATION`

rest-server 0.10.0 introduced a regression which caused the
`DISABLE_AUTHENTICATION` environment variable to stop working for the Docker
container. This has been fixed by automatically setting the option `--no-auth`
to disable authentication.

https://github.com/restic/rest-server/issues/119
https://github.com/restic/rest-server/pull/124