ci: add header with image tags to build-bindgen.sh

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33725>

.gitlab-ci/container/build-bindgen.sh

@@ -1,6 +1,11 @@
 #!/usr/bin/env bash
 # shellcheck disable=SC2086 # we want word splitting
 
+# When changing this file, you need to bump the following
+# .gitlab-ci/image-tags.yml tags:
+# DEBIAN_BUILD_TAG
+# FEDORA_X86_64_BUILD_TAG
+
 uncollapsed_section_start bindgen "Building bindgen"
 
 BINDGEN_VER=0.65.1