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

csp: sso support

See merge request !1

With this change we overwrite the apache2-base php fpm pool config
with our own custom file, which we'll then have to maintain.

Should prevent Roundcube from displaying its own login form when
the SSO ticket expires.

See issue #2 for details. This is a temporary solution.

This reverts commit 931524b4.

Allow all user-owned directories to be bind-mounted by docker (so we
don't have to know which user we'll be running as at runtime).