Location of container files for CI purposes

[Claudio Bantaloukas]

Hi folks,
as part of forge integration, Pietro is proposing that we add a workflow 
that checks autogenerated files are consistent with the autoconf 
versions the project uses.
I've asked Pietro to add his containerfile under 
contrib/ci-container/essential/Containerfile so that this is documented 
and can be used to build the container image used to run the checks on 
the forgejo runner.

Are there any objections on the use of contrib/ci-container/* for 
documenting such containers?

See also https://forge.sourceware.org/gcc/gcc-TEST/pulls/132

Cheers,
Claudio