SSH access to investigate build failures

SSH/shell access, to investigate build failures, would be time-saving for the developer, and resources-saving for salsa-ci (since the dev won't have to re-run a full build to test every single fix attempt) :)

SourceHut has been providing this for a few years:
https://drewdevault.com/2019/08/19/Introducing-shell-access-for-builds.html
https://yotam.net/posts/sourcehut-build/