ci: Rename Containerfile to Dockerfile to fix dependency management

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.