Build autoconf and automake and add autoregen.py from
https://sourceware.org/git/builder.git
Add forge action to build container images.
ChangeLog:
* .forgejo/workflows/build-containers.yaml: New file.
contrib/ChangeLog:
* ci-containers/README: New file.
* ci-containers/autoregen/Containerfile: New file.
* ci-containers/autoregen/autoregen.py: New file.
* ci-containers/build-image.sh: New file.
Signed-off-by: Pietro Monteiro <pietro@sociotechnical.xyz>