This reverts commit 40282123.

This reverts commit d650c370.

Mitigation for contacts having disappeared.

With status code 500 nginx will replace the error page with its own,
and the users won't see the message.

The redirect to the accounts URL was just confusing for users
that hit bugs with the authentication system. It's better to
be explicit about the situation.

A few substantial changes:

* use a multi-stage Dockerfile, instead of a gitlab-CI setup with
  a dedicated build stage (makes the build more easily reproducible)
* ensures permissions are set correctly in the resulting image