added automated build since docker hub wants money if they do it on their side ;D

.github/workflows/build-docker.yml

@@ -0,0 +1,34 @@
+name: opentrashmail ci
+
+# use either
+# ${{steps.date.outputs.date}} for date tag
+# or ${{ github.event.release.tag_name }} for github release
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Get current date
+      id: date
+      run: echo "::set-output name=date::$(date +%s)"
+    - uses: actions/checkout@v2
+    - name: Build the Docker image
+      run: docker build . --file docker/Dockerfile --tag hascheksolutions/opentrashmail:${{ github.event.release.tag_name }} --tag hascheksolutions/opentrashmail:latest
+    -
+      name: Login to registry
+      uses: docker/login-action@v1
+      with:
+        registry: registry.haschek.at
+        username: ${{ secrets.DOCKER_USERNAME }}
+        password: ${{ secrets.DOCKER_PASSWORD }}
+    - name: Pushing push as latest
+      run: docker push hascheksolutions/opentrashmail:latest
+    - name: Pushing to private registry
+      run: docker push hascheksolutions/opentrashmail:${{ github.event.release.tag_name }}