ci: Build container image

Build the container image using GitHub actions, to avoid deployment
failures later on.

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: Tests
+on:
+  push:
+  pull_request:
+    branches: [ main ]
+  schedule:
+    - cron: '0 0 * * *' # Run every day at 00:00 UTC.
+
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/build-push-action@v5
+        with:
+          context: deploy/