improve CI (#6991)

* ci: remove dead BASE_TAGS reference in release bake step

  steps.determine-version doesn't exist in docker-build; the expression
  resolves to empty string. The HCL default (testing) would have
  applied, but it's moot - the bake uses push-by-digest=true so tags are
  only set in merge-manifests. Dead code.

* ci: replace unsecured curl hadolint download with an official action

  hadolint/hadolint-action uses a Docker-based runner with hadolint
  pre-bundled in ghcr.io/hadolint/hadolint:v2.14.0-debian,so no binary
  downloaded at runtime. Pinning the action to a commit SHA covers the
  Dockerfile that specifies the image version, closing the supply-chain
  gap from the previous unverified curl | sudo install.

  Split {debian,alpine}: the action takes a single dockerfile argument,
  so debian and alpine are linted separately.

* ci: pin ubuntu-latest to ubuntu-24.04 in merge-manifests and zizmor

  ubuntu-latest is a moving target that can silently change the runner OS
  on the next GitHub-side update. All other jobs in this repo already pin
  to ubuntu-24.04; this makes merge-manifests and zizmor consistent.

* ci: return BASE_TAGS - it's needed for bake step

.github/workflows/release.yml

@@ -249,7 +249,7 @@ jobs:
 
   merge-manifests:
     name: Merge manifests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     needs: docker-build
     environment: 
       name: release