This patch prepares the switch to Kubernetes 1.24.x which switches to
PSS instead of PSP. Therefore it's a good start to prepare our most
important namespaces with the relevant labels to allow Pods to use
privileged runtime features.

References:
https://kubernetes.io/docs/concepts/security/pod-security-standards/
https://v1-23.docs.kubernetes.io/docs/concepts/security/pod-security-policy/
https://v1-23.docs.kubernetes.io/docs/tasks/configure-pod-container/migrate-from-psp/

This patch prepares the usage of koolbox as release CI image.

This patch pushes the branch reference to the container image tags and
also a semi "latest" tag for each branch, which should help
testing/trying new images.

This patch moves the CLI directory to the koolbox image directory,
bringing it in line with other container images, helping to utilise
standardised workflows.

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.