Follow symlinks inside container root for build's ADD

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

integration/buildfile_test.go

@@ -998,3 +998,21 @@ func TestBuildOnBuildForbiddenMaintainerTrigger(t *testing.T) {
 		t.Fatal("Error should not be nil")
 	}
 }
+
+// gh #2446
+func TestBuildAddToSymlinkDest(t *testing.T) {
+	eng := NewTestEngine(t)
+	defer nuke(mkRuntimeFromEngine(eng, t))
+
+	_, err := buildImage(testContextTemplate{`
+        from {IMAGE}
+        run mkdir /foo
+        run ln -s /foo /bar
+        add foo /bar/
+        run stat /bar/foo
+        `,
+		[][2]string{{"foo", "HEYO"}}, nil}, t, eng, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+}

server/buildfile.go

@@ -395,9 +395,18 @@ func (b *buildFile) checkPathForAddition(orig string) error {
 
 func (b *buildFile) addContext(container *runtime.Container, orig, dest string, remote bool) error {
 	var (
+		err      error
 		origPath = path.Join(b.contextPath, orig)
 		destPath = path.Join(container.RootfsPath(), dest)
 	)
+
+	if destPath != container.RootfsPath() {
+		destPath, err = utils.FollowSymlinkInScope(destPath, container.RootfsPath())
+		if err != nil {
+			return err
+		}
+	}
+
 	// Preserve the trailing '/'
 	if strings.HasSuffix(dest, "/") {
 		destPath = destPath + "/"