Add githubAction to build & push docker image on github release published event. (#881)

fixes #880 👍 

Update Dockerfile
- fix alpine image version
- add RELEASE_VERSION build arg

Add githubAction to build & push docker image on github release published event
- add github workflow file
- add dependabot file to automatically propose PRs on githubActions version update*

> ⚠️ *Note1: you will have to add `DOCKERHUB_USERNAME` and `DOCKERHUB_TOKEN` as github secrets in order for the 
> ⚠️ *Note2: github dependabot support must be enabled in order for this action to work
githubAction to be able to login to the dockerHub and push images on it 👍
> *Note3:  we can add other dependabot entries in order to update Go modules and Dockerfile base images 👍 (i.e `FROM` parts)

.github/workflows/ci-docker.yaml

@@ -0,0 +1,38 @@
+name: ci
+on:
+  release:
+    types: [published]
+jobs:
+  docker-build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: "Checkout repository"
+        uses: actions/checkout@v2
+      -
+        name: "Set up Docker Buildx"
+        uses: docker/setup-buildx-action@v1
+      -
+        name: "Cache Docker layers"
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+      -
+        name: "Login to Container Registry"
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: "Build and push docker image"
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          push: true
+          tags: ${{ github.repository }}:${{ github.event.release.tag_name }}
+          build-args: RELEASE_VERSION=${{ github.event.release.tag_name }}
+          cache-from: type=local,src=/tmp/.buildx-cache
+          cache-to: type=local,mode=max,dest=/tmp/.buildx-cache