ci: build for linux/arm64 (Raspberry Pi 5 target)

Runner is arm64, so native build is much faster than amd64-via-QEMU.
Dev/test amd64 images can still be built locally with 'docker build'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -47,7 +47,9 @@ jobs:
         with:
           context: .
           push: true
-          platforms: linux/amd64
+          # Target: Raspberry Pi 5 (arm64). Runner is arm64 too, so this is native + fast.
+          # amd64 is buildable locally via `docker build` on dev machines.
+          platforms: linux/arm64
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha