setup GitHub action for automatic build of Docker container

as requested in https://github.com/schlagmichdoch/PairDrop/issues/22

.github/workflows/docker-image.yml

@@ -0,0 +1,18 @@
+name: Docker Image CI
+
+on:
+  push:
+    branches: [ "master" ]
+  pull_request:
+    branches: [ "master" ]
+
+jobs:
+
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Build the Docker image
+      run: docker build . -f Dockerfile -t pairdrop