migrate to upstream libcompose in one and a half go

vendor/github.com/docker/distribution/context/doc.go

@@ -3,6 +3,19 @@
 // logging relevent request information but this package is not limited to
 // that purpose.
 //
+// The easiest way to get started is to get the background context:
+//
+// 	ctx := context.Background()
+//
+// The returned context should be passed around your application and be the
+// root of all other context instances. If the application has a version, this
+// line should be called before anything else:
+//
+// 	ctx := context.WithVersion(context.Background(), version)
+//
+// The above will store the version in the context and will be available to
+// the logger.
+//
 // Logging
 //
 // The most useful aspect of this package is GetLogger. This function takes