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>

.gitea/scripts/resolve-base-image.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+## resolve-base-image.sh IMAGE:TAG — CI only (not copied into images).
+## Prints IMAGE@sha256:<digest> for a Docker Hub tag, looked up DIRECTLY at
+## registry-1.docker.io over HTTPS (never via the pkg-cache mirror), so the
+## build can say `FROM IMAGE@sha256:...` and the mirror can only serve
+## content-addressed blobs that BuildKit verifies. Exits non-zero (prints
+## nothing on stdout) if the lookup fails; the workflow then skips the mirror
+## and pulls the tag directly from Docker Hub. HEAD requests do not count
+## against Docker Hub pull rate limits.
+## Canonical copy: cloud-apache-container/.gitea/scripts/resolve-base-image.sh
+## (docs/pkg-cache.md there); other repos carry identical copies.
+set -euo pipefail
+ref="${1:?usage: resolve-base-image.sh IMAGE:TAG}"
+name_re='^[a-z0-9]+([._-][a-z0-9]+)*(/[a-z0-9]+([._-][a-z0-9]+)*)?$'
+tag_re='^[A-Za-z0-9_][A-Za-z0-9_.-]{0,127}$'
+name="${ref%:*}"; tag="${ref##*:}"
+[ "$name" != "$ref" ] || { echo "resolve-base-image: '$ref' has no :tag" >&2; exit 2; }
+[[ "$name" =~ $name_re ]] || { echo "resolve-base-image: bad image name '$name'" >&2; exit 2; }
+[[ "$tag" =~ $tag_re ]] || { echo "resolve-base-image: bad tag '$tag'" >&2; exit 2; }
+repo="$name"; [[ "$repo" == */* ]] || repo="library/$repo"   # official images
+
+curl_opts=(-fsS --proto '=https' --retry 3 --retry-all-errors --max-time 20)
+token=$(curl "${curl_opts[@]}" "https://auth.docker.io/token?service=registry.docker.io&scope=repository:${repo}:pull" |
+        python3 -c 'import json,sys; print(json.load(sys.stdin)["token"])')
+[ -n "$token" ] || { echo "resolve-base-image: no Docker Hub token for $repo" >&2; exit 1; }
+hdrs=$(curl "${curl_opts[@]}" -I \
+    -H "Authorization: Bearer ${token}" \
+    -H 'Accept: application/vnd.oci.image.index.v1+json' \
+    -H 'Accept: application/vnd.docker.distribution.manifest.list.v2+json' \
+    -H 'Accept: application/vnd.oci.image.manifest.v1+json' \
+    -H 'Accept: application/vnd.docker.distribution.manifest.v2+json' \
+    "https://registry-1.docker.io/v2/${repo}/manifests/${tag}")
+digest=$(printf '%s\n' "$hdrs" | tr -d '\r' | awk 'tolower($1)=="docker-content-digest:" {print $2}' | tail -n1)
+[[ "$digest" =~ ^sha256:[0-9a-f]{64}$ ]] || { echo "resolve-base-image: no valid digest for $ref (got '${digest}')" >&2; exit 1; }
+printf '%s@%s\n' "$name" "$digest"