Fix up the container build to use a multi-stage build, cache better, and most importantly actually keep the library in the final image (fixes #931)

.github/workflows/container-build.yaml

@@ -75,3 +75,5 @@ jobs:
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max