Add container build

Author: theseanything

.github/workflows/ci.yml

@@ -112,6 +112,10 @@ jobs:
       security-events: write
       actions: read
 
+  build-image:
+    name: Build image
+    uses: alphagov/forms-deploy/.github/workflows/reusable-build-image.yml@main
+
   audit-dependencies:
     name: Audit dependencies
     runs-on: ubuntu-latest