docker: add workflow to test image works

Because it is broken at the moment.

snap/bin/snapcraft: 3: exec: /snap/snapcraft/current/usr/bin/python3: not found

.github/workflows/docker-image.yml

@@ -0,0 +1,20 @@
+name: Docker Image CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Build the Docker image
+      run: docker build . --file docker/Dockerfile --tag snapcraft:test
+    - name: Test the image works
+      run: docker run snapcraft:test snapcraft version