ci: use shellcheck for .gitlab-ci/container/ directory

It checks our CI shell code in `debian-build-testing` job.

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>

.gitlab-ci/run-shellcheck.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+CHECKPATH=".gitlab-ci/container"  # TODO: expand to cover whole .gitlab-ci/
+
+is_bash() {
+    [[ $1 == *.sh ]] && return 0
+    [[ $1 == */bash-completion/* ]] && return 0
+    [[ $(file -b --mime-type "$1") == text/x-shellscript ]] && return 0
+    return 1
+}
+
+while IFS= read -r -d $'' file; do
+    if is_bash "$file" ; then
+        shellcheck -x -W0 -s bash "$file"
+        rc=$?
+        if [ "${rc}" -eq 0 ]
+        then
+            continue
+        else
+            exit 1
+        fi
+    fi
+done < <(find $CHECKPATH -type f \! -path "./.git/*" -print0)