migrate to upstream libcompose in one and a half go

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

@@ -0,0 +1,19 @@
+package context
+
+import "testing"
+
+func TestVersionContext(t *testing.T) {
+	ctx := Background()
+
+	if GetVersion(ctx) != "" {
+		t.Fatalf("context should not yet have a version")
+	}
+
+	expected := "2.1-whatever"
+	ctx = WithVersion(ctx, expected)
+	version := GetVersion(ctx)
+
+	if version != expected {
+		t.Fatalf("version was not set: %q != %q", version, expected)
+	}
+}