Move to its own file and only trigger on release

The reason for this is that the build is that the version is
taken from gradle so if we built on any other action it would
result in a constant overwrite of the image.

.github/workflows/ci-docker.yml

@@ -0,0 +1,46 @@
+name: signal-cli Docker Image
+
+on:
+  release:
+    types: [published]
+
+jobs:
+
+  build-docker:
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Set up JDK
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'adopt'
+          java-version: '17'
+          cache: 'gradle'
+
+      - name: Build with Gradle
+        run: ./gradlew --no-daemon build
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Get current version from gradle
+        run: echo "APP_VERSION=$(./gradlew --no-daemon properties -q | grep "version:" | awk '{print $2}')" >> $GITHUB_ENV
+
+      - name: Build and push
+        uses: docker/build-push-action@v3
+        with:
+          context: .
+          push: true
+          tags: ${{ secrets.DOCKERHUB_USERNAME }}/signal-cli:latest, ${{ secrets.DOCKERHUB_USERNAME }}/signal-cli:${{ env.APP_VERSION }}