ci: Enable ccache in the container builds.

This should reduce our container rebuild times, particularly on the
40-minute ARM build (which is split across only 2 runners and thus likely
to have a hot cache) when working on updating containers.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4099>

.gitlab-ci/container/container_pre_build.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+export CCACHE_COMPILERCHECK=content
+export CCACHE_COMPRESS=true
+export CCACHE_DIR=/cache/mesa/ccache
+export PATH=/usr/lib/ccache:$PATH
+
+ccache --show-stats