hack/build-image.sh: Update Dockerfile reference

The file named `Dockerfile` was removed in the [1] commit as
the file was unused. Update the outdated reference to point at
the sole existing Dockerfile file in the repository.

[1]: 2f996d4

Author: DavidHurta

hack/build-image.sh

@@ -22,4 +22,4 @@ if [ -z ${VERSION_OVERRIDE+a} ]; then
 fi
 
 set -x
-podman build -t "cluster-version-operator:${VERSION_OVERRIDE}" -f Dockerfile --no-cache
+podman build -t "cluster-version-operator:${VERSION_OVERRIDE}" -f Dockerfile.rhel --no-cache