build: publish amd64 only

The NAS is the only host this container runs on. Building linux/arm64 as well
meant emulating it under QEMU on every release for a consumer that does not
exist, so both the second platform and the QEMU setup step are dropped.

Nothing in the image is architecture-specific; restoring arm64 is a one-line
change if it ever gains a home on the Pi.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-publish.yaml

@@ -118,9 +118,6 @@ jobs:
           echo "release=${release}" >> "$GITHUB_OUTPUT"
           echo "Computed bump=${bump}, release=${release}, base=${base}"
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@06116385d9baf250c9f4dcb4858b16962ea869c3 # v4
-
       - name: Set up Buildx
         uses: docker/setup-buildx-action@d7f5e7f509e45cec5c76c4d5afdd7de93d0b3df5 # v4
 
@@ -139,8 +136,9 @@ jobs:
           # Without this the last stage in the Dockerfile -- the test stage --
           # would be what gets published.
           target: runtime
-          # amd64 for the NAS, arm64 so the same image runs on a Pi.
-          platforms: linux/amd64,linux/arm64
+          # The NAS is the only host this runs on. Building arm64 as well would
+          # mean emulating it under QEMU for no consumer.
+          platforms: linux/amd64
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.version.outputs.tags }}
           labels: |