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>

docker/act_runner.config.yaml

@@ -0,0 +1,16 @@
+runner:
+  capacity: 1
+  timeout: 3h
+  insecure: false
+
+cache:
+  enabled: true
+
+container:
+  # Pass the host Docker socket into every job container so
+  # docker/build-push-action can build and push images.
+  options: -v /var/run/docker.sock:/var/run/docker.sock
+  # Use the host network so the job container can reach forge.dilain.com
+  network: host
+  valid_volumes:
+    - /var/run/docker.sock