build: configure act_runner and simplify Docker workflow to amd64

- act_runner.compose.yml: auto-register with forge, mount Docker socket
- act_runner.config.yaml: pass Docker socket to job containers, host network
- docker.yml: drop QEMU/multi-arch, build linux/amd64 only (faster)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker.yml

@@ -12,9 +12,6 @@ jobs:
       - name: checkout
         uses: actions/checkout@v3
 
-      - name: qemu
-        uses: docker/setup-qemu-action@v2
-
       - name: buildx
         uses: docker/setup-buildx-action@v2
 
@@ -30,7 +27,7 @@ jobs:
         with:
           context: .
           file: ./docker/Dockerfile
-          platforms: linux/amd64,linux/arm64
+          platforms: linux/amd64
           push: true
           cache-from: type=gha
           cache-to: type=gha,mode=max