ci: make shellcheck happy about .gitlab-ci/container/ directory

Makes easier do changes, when shellcheck is warning-free.

Reviewed-by: Guilherme Gallo <guilherme.gallo@collabora.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17574>
28 files changed