Given that currently a lot of connections look broken and result quite delayed
mail delivery adjusting this interval will hopefully help to keep imap
connections alive and therefore delivering emails in realtime.

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

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.

This patch reduces the image builds to a minimum stable releases should
only be build then the `.release` file changes, while deb builds can run
on any commit.

This patch adds a gitlab-ci integration while tekton is still in the
works. This should build all images required, while keeping the door
open for switching to tekton soon.