fix(ci-images): static build matrix for Gitea

Gitea expands a job's matrix when the run is created, before plan has any
outputs, so fromJSON(needs.plan.outputs.matrix) collapsed to a single empty
"Build ${{ matrix.key }}" job and nothing was ever built. The matrix is now
the fixed list of image keys; plan emits every image's spec with a build flag
and each matrix job looks its own entry up, no-opping when it wasn't picked.

Also document that both registry tokens need write:package -- the vault token
was read-only, so the gitea_ci_build_local bootstrap failed its push.

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

.gitea/workflows/ci-images.yml

@@ -63,7 +63,7 @@ jobs:
     name: Plan
     runs-on: fedora
     outputs:
-      matrix: ${{ steps.plan.outputs.matrix }}
+      images: ${{ steps.plan.outputs.images }}
       any: ${{ steps.plan.outputs.any }}
     steps:
       - uses: actions/checkout@v4
@@ -153,7 +153,12 @@ jobs:
                   else:
                       picked = [i for i in images if f"{ctx}/{i['containerfile']}" in touched]
 
-          print(f"matrix={json.dumps({'include': picked})}")
+          # Every image, keyed by matrix.key, each flagged build or skip. The
+          # build job's matrix is static (see there), so it needs the full set
+          # to look its own entry up in, not just the picked ones.
+          picked_keys = {i["key"] for i in picked}
+          specs = {i["key"]: {**i, "build": i["key"] in picked_keys} for i in images}
+          print(f"images={json.dumps(specs)}")
           print(f"any={'true' if picked else 'false'}")
           print("building: " + (", ".join(i["tag"] for i in picked) or "nothing"), file=sys.stderr)
           PY
@@ -167,11 +172,34 @@ jobs:
       # One image failing must not cancel the others: they are independent, and
       # a half-published set is what this whole workflow exists to avoid.
       fail-fast: false
-      matrix: ${{ fromJSON(needs.plan.outputs.matrix) }}
+      # Static on purpose. Gitea expands the matrix when the run is created,
+      # before plan has produced any outputs, so a
+      # fromJSON(needs.plan.outputs.*) matrix collapses to one empty job. Each
+      # entry instead looks its spec up in plan's output and no-ops its steps
+      # when plan did not pick it. Keys must match `images` in plan.
+      matrix:
+        key: [ci, espidf, platformio]
     steps:
+      - name: Look up the ${{ matrix.key }} image spec
+        id: spec
+        env:
+          IMAGES: ${{ needs.plan.outputs.images }}
+          KEY: ${{ matrix.key }}
+        run: |
+          set -euo pipefail
+          python3 - <<'PY' >> "$GITHUB_OUTPUT"
+          import json, os
+          spec = json.loads(os.environ["IMAGES"])[os.environ["KEY"]]
+          for k in ("containerfile", "tag", "build_args"):
+              print(f"{k}={spec[k]}")
+          print(f"build={'true' if spec['build'] else 'false'}")
+          PY
+
       - uses: actions/checkout@v4
+        if: steps.spec.outputs.build == 'true'
 
       - name: Log in to the Gitea Container Registry
+        if: steps.spec.outputs.build == 'true'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
@@ -184,18 +212,20 @@ jobs:
       # image, so PRs get a throwaway tag that the cleanup step removes.
       - name: Resolve build tag
         id: tag
+        if: steps.spec.outputs.build == 'true'
         run: |
           set -euo pipefail
           if [ "${{ github.event_name }}" = "pull_request" ]; then
-            echo "image=${{ matrix.tag }}-pr${{ github.event.number }}" >> "$GITHUB_OUTPUT"
+            echo "image=${{ steps.spec.outputs.tag }}-pr${{ github.event.number }}" >> "$GITHUB_OUTPUT"
           else
-            echo "image=${{ matrix.tag }}" >> "$GITHUB_OUTPUT"
+            echo "image=${{ steps.spec.outputs.tag }}" >> "$GITHUB_OUTPUT"
           fi
 
       - name: Build ${{ matrix.key }}
+        if: steps.spec.outputs.build == 'true'
         env:
           IMAGE: ${{ steps.tag.outputs.image }}
-          BUILD_ARGS: ${{ matrix.build_args }}
+          BUILD_ARGS: ${{ steps.spec.outputs.build_args }}
         run: |
           set -euo pipefail
           args=()
@@ -205,10 +235,11 @@ jobs:
           docker build --pull \
             "${args[@]}" \
             -t "$IMAGE" \
-            -f "$CONTEXT/${{ matrix.containerfile }}" \
+            -f "$CONTEXT/${{ steps.spec.outputs.containerfile }}" \
             "$CONTEXT"
 
       - name: Verify the prune-exemption label survived the build
+        if: steps.spec.outputs.build == 'true'
         env:
           IMAGE: ${{ steps.tag.outputs.image }}
         run: |
@@ -224,7 +255,7 @@ jobs:
           }
 
       - name: Push ${{ matrix.key }}
-        if: github.event_name != 'pull_request'
+        if: github.event_name != 'pull_request' && steps.spec.outputs.build == 'true'
         env:
           IMAGE: ${{ steps.tag.outputs.image }}
         run: |
@@ -236,7 +267,7 @@ jobs:
       # so the nightly prune will not reclaim it and a few skipped cleanups add
       # up to gigabytes in the runner's store.
       - name: Drop the pull-request image
-        if: always() && github.event_name == 'pull_request'
+        if: always() && github.event_name == 'pull_request' && steps.spec.outputs.build == 'true'
         env:
           IMAGE: ${{ steps.tag.outputs.image }}
         run: docker rmi -f "$IMAGE" || true