Provide a gitlab-runner configuration that can be used for non-cloud runners and compatible with unshare

Build arm64 jobs have started to fail on (perhaps needs --mode=root?):

$ su salsa-ci -c "mmdebstrap --skip=output/dev --variant=buildd ${MMDEBSTRAP_INCLUDE_PACKAGES_ARGS} --components=\"${MMDEBSTRAP_COMPONENTS}\" ${MMDEBSTRAP_CUSTOMIZE_HOOK_ARGS} ${SALSA_CI_MMDEBSTRAP_EXTRA_ARGS} ${mmdebstrap_dist} ~/.cache/sbuild/${mmdebstrap_dist}-amd64.tar ${SALSA_CI_MIRROR}"
E: unable to pick chroot mode automatically (use --mode for manual selection)

And later, since after_script always runs, it too fails on:

$ test ! "${ENABLE_CCACHE}" != "0" || mv -v ${CCACHE_TMP_DIR} ${CCACHE_WORK_DIR}
mv: cannot stat '/builds/DebianOnMobile-team/megapixels/../.megapixels.ccache': No such file or directory

Examples:
https://salsa.debian.org/DebianOnMobile-team/megapixels/-/jobs/8292394
https://salsa.debian.org/Mobian-team/packages/unl0kr/-/jobs/8294700

For more related jobs, see: https://salsa-status.debian.net/job-types/build%20arm64