This patch reworks the image build process, to utilise GitLab matrix
build feature to generate all jobs, reducing code duplication.

This patch fixes the absense of persistence for sieve rules, due to
being layed out in parallel to the `/srv/mail/mailbox` directory in the
dovecot container.

By moving the mountpoint to `/var/mail` the sieve rules should be
persisted along the way with the mailboxes.

BREAKING CHANGE: This patch adjusts the mount location for mail data,
which will require manual interaction.

This patch configures the longhorn backup storage for longhorn. It's a
first run until the proper backup storage is complete.

This patch renames all uses of `Containerfile` with `Dockerfile` in
order to enable renovate to manage dependencies and images properly.
Currently renovate doesn't support the Containerfile name as match for
its docker module, which would mean that either there is custom config
added or renaming the files. Since there is no explicit benefit of using
Containerfile as name, this patch just renames them in order to avoid
custom renovate config.

Turns out there are unexpected structural changes in this upgrade that
we need to address separately.

It requires to deploy a CR after successful deployment, which means
we'll have to move the helm chart to bootstrape of work around this in
another way.

This patch adds the missing capability to install CRDs as part of the
metallb deployement.

This patch removes the component label, which was previously manually
added by our deployment, but is now part of the helm chart.