Detect if images are available if we don't have to generate a Package

While building, if we aren't doing a clean build, we scan now to see if
images are available and we skip, in case we don't find a metadata
already.

pkg/compiler/backend/simpledocker_test.go

@@ -124,5 +124,11 @@ RUN echo bar > /test2`))
 
 		})
 
+		It("Detects available images", func() {
+			b := NewSimpleDockerBackend()
+			Expect(b.ImageAvailable("quay.io/mocaccino/extra")).To(BeTrue())
+			Expect(b.ImageAvailable("ubuntu:20.10")).To(BeTrue())
+			Expect(b.ImageAvailable("igjo5ijgo25nho52")).To(BeFalse())
+		})
 	})
 })