ci(docker): use registry-backed build cache for arm64 (#37129)

The arm64 PR build ran fully uncached because the previous gha cache
backend's short-lived Azure SAS token expired mid-build on slow
cold-cache arm64 runs and crashed before the smoke test. Uncached arm64
PR builds were ~45% slower than amd64 (median 553s vs 382s), making the
arm64 job the one most often cancelled on supersede — surfacing as a red
X in PR checks and reading as 'the arm64 build keeps failing'.

Switch arm64 to a registry-backed cache on ghcr.io
(type=registry, ref ghcr.io/nousresearch/hermes-agent:buildcache-arm64).
Its credential is the job-lifetime GITHUB_TOKEN, not a time-boxed SAS
token, so the cold-build-outlives-token failure mode cannot recur.

- PR builds: cache-from only (read-only) — warm layers, no write races,
  no cache-ref pollution from rapid PR pushes.
- main/release builds: cache-from + cache-to (mode=max) to populate the
  cache for subsequent PR/main builds and let the digest push reuse the
  smoke-test build's layers.
- Add packages: write permission and a ghcr.io login for the cache.

amd64 keeps its gha cache: it builds fast enough to stay inside the SAS
token's lifetime, so it never hit this failure mode.

.github/workflows/docker-publish.yml

@@ -26,6 +26,10 @@ on:
 
 permissions:
   contents: read
+  # Needed so the arm64 job can push/pull its registry-backed build cache
+  # to ghcr.io (cache-to/cache-from type=registry).  See the build-arm64
+  # job for why registry cache replaced the gha cache on that arch.
+  packages: write
 
 # Concurrency: push/release runs are NEVER cancelled so every merge gets
 # its own image.  PR runs reuse a PR-scoped group with
@@ -196,11 +200,34 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f  # v3
 
-      # Build once, load into the local daemon for smoke testing. PR arm64
-      # builds deliberately avoid the gha cache: cold-cache arm64 builds can
-      # outlive GitHub's short-lived Azure cache SAS token, then fail while
-      # reading or writing cache blobs before the smoke test can run.
-      - name: Build image (arm64, smoke test, uncached PR)
+      # Log in to ghcr.io so the registry-backed build cache below can be
+      # read (cache-from) on every event and written (cache-to) on
+      # push/release.  Uses the workflow's GITHUB_TOKEN, which is valid for
+      # the whole job — unlike the gha cache backend's short-lived Azure SAS
+      # token, which expired mid-build on slow cold-cache arm64 runs and
+      # crashed the build before the smoke test (the reason the gha cache
+      # was removed from arm64 PRs in the first place).
+      - name: Log in to ghcr.io (build cache)
+        uses: docker/login-action@4907a6ddec9925e35a0a9e82d7399ccc52663121  # v4.1.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Build once, load into the local daemon for smoke testing.
+      #
+      # PR builds use the registry-backed cache READ-ONLY (cache-from only):
+      # they pull warm layers pushed by the most recent main build but never
+      # write, so rapid PR pushes don't race on cache writes or pollute the
+      # cache ref.  This restores warm-cache speed to arm64 PR builds (which
+      # were running fully uncached and were ~45% slower than amd64, making
+      # them the job most often cancelled on supersede).
+      #
+      # Registry cache (type=registry on ghcr.io) is used instead of the gha
+      # cache that previously broke here: its credential is the job-lifetime
+      # GITHUB_TOKEN, not a short-lived SAS token, so the cold-build-outlives-
+      # token failure mode cannot recur.
+      - name: Build image (arm64, smoke test, cache read-only PR)
         if: github.event_name == 'pull_request'
         uses: docker/build-push-action@bcafcacb16a39f128d818304e6c9c0c18556b85f  # v7.1.0
         with:
@@ -211,9 +238,11 @@ jobs:
           tags: ${{ env.IMAGE_NAME }}:test
           build-args: |
             HERMES_GIT_SHA=${{ github.sha }}
+          cache-from: type=registry,ref=ghcr.io/nousresearch/hermes-agent:buildcache-arm64
 
-      # Main/release builds still use the per-arch gha cache so the digest
-      # push below can reuse layers from this smoke-test build.
+      # Main/release builds read AND write the registry cache so the digest
+      # push below reuses layers from this smoke-test build, and so the next
+      # PR/main build starts warm.
       - name: Build image (arm64, smoke test, cached publish)
         if: github.event_name != 'pull_request'
         uses: docker/build-push-action@bcafcacb16a39f128d818304e6c9c0c18556b85f  # v7.1.0
@@ -225,8 +254,8 @@ jobs:
           tags: ${{ env.IMAGE_NAME }}:test
           build-args: |
             HERMES_GIT_SHA=${{ github.sha }}
-          cache-from: type=gha,scope=docker-arm64
-          cache-to: type=gha,mode=max,scope=docker-arm64
+          cache-from: type=registry,ref=ghcr.io/nousresearch/hermes-agent:buildcache-arm64
+          cache-to: type=registry,ref=ghcr.io/nousresearch/hermes-agent:buildcache-arm64,mode=max
 
       - name: Smoke test image
         uses: ./.github/actions/hermes-smoke-test
@@ -253,8 +282,8 @@ jobs:
           build-args: |
             HERMES_GIT_SHA=${{ github.sha }}
           outputs: type=image,name=${{ env.IMAGE_NAME }},push-by-digest=true,name-canonical=true,push=true
-          cache-from: type=gha,scope=docker-arm64
-          cache-to: type=gha,mode=max,scope=docker-arm64
+          cache-from: type=registry,ref=ghcr.io/nousresearch/hermes-agent:buildcache-arm64
+          cache-to: type=registry,ref=ghcr.io/nousresearch/hermes-agent:buildcache-arm64,mode=max
 
       - name: Export digest
         if: github.event_name == 'push' && github.ref == 'refs/heads/main' || github.event_name == 'release'