gha: add docker publish action

.github/workflows/docker-publish.yml

@@ -0,0 +1,34 @@
+name: Publish Docker image
+
+on:
+  schedule:
+    - cron: 0 0 1 * * # First day on every month
+
+jobs:
+  push_to_registry:
+    name: Push Docker image to GitHub Packages
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          file: ./Dockerfile
+          push: true
+          tags: frealmyr/platformio:latest
+
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}