[gha] build docker containers

Build docker containers and publish them to GitHub Container Registry

.github/workflows/docker.yml

@@ -0,0 +1,28 @@
+name: Build Docker containers
+
+on: push
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to DockerHub
+        uses: docker/login-action@v1 
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: contrib/docker/Dockerfile
+          platforms: linux/amd64,linux/386,linux/arm/v7,linux/arm64,linux/ppc64le
+          push: true
+          tags: ghcr.io/PurpleI2P/i2pd:latest