ci(actions): add build docker image workflow (#118)

### Add secret
```
DOCKERHUB_TOKEN=xxx
```

### Tag
when tag like `v1.0.0`, it will build multi platform docker image `gitea/act_runner:1.0.0` and `gitea/act_runner:latest`, then push to docker hub

### Use
> volume `/data` save `.runner` config file
> volume `/root/.cache` save actcache and actions cache

```sh
docker run -e GITEA_INSTANCE_URL=***                    \
           -e GITEA_RUNNER_REGISTRATION_TOKEN=***       \
           -e GITEA_RUNNER_NAME=***                     \
           -v /var/run/docker.sock:/var/run/docker.sock \
           -v /root/act_runner/data:/data               \
           -v /root/act_runner/cache:/root/.cache       \
           gitea/act_runner
```
Test join runners success
![image](/attachments/f5287e93-e27c-420f-a3d5-8f9b54bfdbb6)
Test run action success
![image](/attachments/7235af17-f598-4fc8-88b4-d4771b1f07cd)

Co-authored-by: Jason Song <i@wolfogre.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/118
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: seepine <seepine@noreply.gitea.io>
Co-committed-by: seepine <seepine@noreply.gitea.io>

.gitea/workflows/build-image.yml

@@ -0,0 +1,46 @@
+name: Build image
+on:
+  push:
+    tags:
+      - v*
+jobs:
+  build-image:
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    env:
+      DOCKER_USERNAME: gitea
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+
+      - name: Set up Docker BuildX
+        uses: docker/setup-buildx-action@v2
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ env.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Get Meta
+        id: meta
+        run: |
+          echo REPO_NAME=$(echo ${GITHUB_REPOSITORY} | awk -F"/" '{print $2}') >> $GITHUB_OUTPUT
+          echo REPO_VERSION=$(echo ${{ github.ref }} | awk -F"/" '{print $3}' | awk -F"v" '{print $2}') >> $GITHUB_OUTPUT          
+
+      - name: Build and push
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: |
+            linux/amd64
+            linux/arm64            
+          push: true
+          tags: |
+            ${{ env.DOCKER_USERNAME }}/${{ steps.meta.outputs.REPO_NAME }}:latest
+            ${{ env.DOCKER_USERNAME }}/${{ steps.meta.outputs.REPO_NAME }}:${{ steps.meta.outputs.REPO_VERSION }}