Enable and set up Docker Buildx for multi-platform builds

Also add caching of Go modules

.github/workflows/release.yaml

@@ -10,19 +10,35 @@ jobs:
   release:
     name: Release
     runs-on: ubuntu-18.04 # 20.04 doesn't provide createrepo for now
+    env:
+      DOCKER_CLI_EXPERIMENTAL: "enabled"
     steps:
       - name: Install dependencies
         run: |
           sudo apt-get -y update
           sudo apt-get -y install rpm reprepro createrepo distro-info
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Show available Docker Buildx platforms
+        run: echo ${{ steps.buildx.outputs.platforms }}
       - name: Setup Go
-        uses: actions/setup-go@v1
+        uses: actions/setup-go@v2
         with:
           go-version: ${{ env.GO_VERSION }}
       - name: Checkout code
         uses: actions/checkout@v2
         with:
           fetch-depth: 0
+      - name: Cache Go modules
+        uses: actions/cache@v2
+        with:
+          path: ~/go/pkg/mod
+          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-
       # - name: Checkout trivy-repo
       #   uses: actions/checkout@v2
       #   with: