ci: document which image tags need to be bumped when updating {alpine,debian,fedora}/x86_64

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25120>

.gitlab-ci/container/fedora/x86_64_build.sh

@@ -1,5 +1,10 @@
 #!/usr/bin/env bash
 # shellcheck disable=SC1091
+
+# When changing this file, you need to bump the following
+# .gitlab-ci/image-tags.yml tags:
+# FEDORA_X86_64_BUILD_TAG
+
 set -e
 set -o xtrace