Add PreBuild steps

They are executed when building the "builder" image, those are not accounted to the diffs when producing the final artifact

pkg/compiler/interface.go

@@ -45,4 +45,10 @@ type CompilationSpec interface {
 
 	GetImage() string
 	SetImage(string)
+
+	SetOutputPath(string)
+	GetOutputPath() string
+	Rel(string) string
+
+	GetPreBuildSteps() []string
 }