Add action to build and push docker image on master update

Signed-off-by: Waqar Ahmed <waqarahmedjoyia@live.com>

.github/workflows/docker_image.yml

+name: build_image
+
+on:
+  push:
+    branches:
+      - 'truenas/zfs-2.1-release'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - 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:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          tags: ixsystems/zfs:latest
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}