support ccache on local disk during container build
For normal build jobs on our shared runners, there's a /cache
directory that's volume-mounted to the host where we get to stick ccache contents so that they're shared between all pipelines for Mesa. For the ci-templates containers builds, it would be really nice to also have the /cache directory present so we could have ccache during the container builds -- right now Mesa's container build process for the arm-build container takes 50 minutes uncached and sometimes times out (https://gitlab.freedesktop.org/anholt/mesa/-/jobs/1606281 for example). If we were able to have ccache in the build that would speed things up significantly during development.
All I think I would need is the ability to specify a volume to be mounted during $DEBIAN_EXEC (/cache:/cache would be what I need).