Add a github workflow to build the container image

The workflow builds the container image using the Containerfile in the repo;
the Containerfile runs the tests as part of the build process.

Author: owtaylor

.github/workflows/build-container.yml

@@ -0,0 +1,27 @@
+name: Build Container Image
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - name: Build container image with podman
+        run: |
+          podman build \
+              -f Containerfile \
+              -t flatpak-indexer:latest \
+              --build-arg=FLATPAK_INDEXER_UPDATE_TEST_DATA=1 \
+              .