Create workflow for building cross-platform image

.github/workflows/cross-compile-docker.yml

@@ -0,0 +1,38 @@
+name: "(Cross) Compile Dendrite"
+
+# dummy
+on: push
+
+jobs:
+  compile:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        binary: [dendrite-monolith-server]
+    env:
+      GOARCH: ${{ matrix.target }}
+      DOCKER_HUB_USER: dendritegithub
+      # DOCKER_NAMESPACE: matrixdotorg
+      DOCKER_NAMESPACE: localhost:5000/matrixdotorg
+      PLATFORMS: linux/amd64,linux/arm64,linux/arm/v7
+    services:
+      registry:
+        image: registry:2
+        ports:
+        - 5000:5000
+    steps:
+    # - run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+    - uses: actions/checkout@v2
+    # - uses: docker/setup-qemu-action@v1
+    - uses: docker/setup-buildx-action@v1
+      with:
+        driver-opts: network=host
+    - name: build dendrite in Docker
+      uses: docker/build-push-action@v2
+      with:
+        context: .
+        file: ./build/docker/Dockerfile
+        push: true
+        platforms: ${{ env.PLATFORMS }}
+        build-args: "DENDRITE_BINARY=${{ matrix.binary }}"
+        tags: "${{ env.DOCKER_NAMESPACE }}/${{ matrix.binary }}:latest"