ci: pin base images by digest resolved directly from Docker Hub

With the pkg-cache BuildKit mirror on, `FROM almalinux/10-base` was resolved
by tag through the cache, so a compromised cache could substitute the image.

Each job using the mirror now looks up the base tag's digest DIRECTLY at
registry-1.docker.io (.gitea/scripts/resolve-base-image.sh, HTTPS, HEAD on
the manifest, regex-validated) and builds FROM image@sha256:... via the
BASE_IMAGE build-arg, so the mirror can only serve content-addressed blobs.
If the lookup fails, the mirror is NOT used for that job and the tag is pulled
straight from Docker Hub. The digest is resolved every run (no manual bumps);
local builds default to the tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile.shared-httpd

@@ -1,4 +1,9 @@
-FROM almalinux/10-base
+# Base image. CI passes BASE_IMAGE=almalinux/10-base@sha256:<digest>, resolved
+# DIRECTLY from Docker Hub in the same run (.gitea/scripts/resolve-base-image.sh),
+# so the pkg-cache registry mirror can only serve content-addressed blobs.
+# Local builds use the tag. docs/pkg-cache.md "Base-image digest pinning".
+ARG BASE_IMAGE=almalinux/10-base
+FROM ${BASE_IMAGE}
 
 # OPTIONAL internal package cache, e.g. --build-arg PKG_CACHE=https://192.168.1.40
 # (CT130 pkg-cache, https only). Unset = upstream repositories. Release RPMs are