Pin registry auth realm host for shared CI image jobs

.gitea/workflows/cicd.yaml

@@ -563,6 +563,27 @@ jobs:
       - &configure_registry_host_step
         name: Configure registry host resolution
         run: |
+          ensure_registry_auth_realm_host() {
+            if ! grep -q "${GITEA_REGISTRY_HOST}" /etc/hosts; then
+              echo "${GITEA_REGISTRY_IP} ${GITEA_REGISTRY_HOST}" >> /etc/hosts
+            fi
+
+            headers_file="$(mktemp)"
+            if curl -sSI "http://${GITEA_REGISTRY}/v2/" >"${headers_file}"; then
+              realm_url="$(sed -n 's/.*realm="\([^"]*\)".*/\1/p' "${headers_file}" | head -n 1)"
+              if [ -n "${realm_url}" ]; then
+                realm_host="$(printf '%s' "${realm_url}" | sed -E 's#^https?://([^/:]+).*$#\1#')"
+                if [ -n "${realm_host}" ] && [ "${realm_host}" != "${GITEA_REGISTRY_HOST}" ]; then
+                  if ! grep -q "${realm_host}" /etc/hosts; then
+                    echo "${GITEA_REGISTRY_IP} ${realm_host}" >> /etc/hosts
+                    echo "Pinned registry auth realm host: ${realm_host} -> ${GITEA_REGISTRY_IP}"
+                  fi
+                fi
+              fi
+            fi
+            rm -f "${headers_file}"
+          }
+
           if [ "${CI_PRUNE_AT_JOB_START:-false}" = "true" ] && command -v docker >/dev/null 2>&1; then
             used_before="$(df -P / | awk 'NR==2 {gsub(/%/, "", $5); print $5}')"
             threshold="${CI_PRUNE_THRESHOLD_PERCENT:-90}"
@@ -582,6 +603,8 @@ jobs:
             echo "${GITEA_REGISTRY_IP} ${GITEA_REGISTRY_HOST}" >> /etc/hosts
           fi
 
+          ensure_registry_auth_realm_host
+
       - &ensure_cicd_image_step
         name: Ensure CICD image is available
         env: