Add proper support for relative WORKDIR instructions

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

integration/buildfile_test.go

@@ -441,6 +441,23 @@ func TestBuildUser(t *testing.T) {
 	}
 }
 
+func TestBuildRelativeWorkdir(t *testing.T) {
+	img, err := buildImage(testContextTemplate{`
+		FROM {IMAGE}
+		RUN [ "$PWD" = '/' ]
+		WORKDIR /test1
+		RUN [ "$PWD" = '/test1' ]
+		WORKDIR test2
+		RUN [ "$PWD" = '/test1/test2' ]
+	`, nil, nil}, t, nil, true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if img.Config.WorkingDir != "/test1/test2" {
+		t.Fail()
+	}
+}
+
 func TestBuildEnv(t *testing.T) {
 	img, err := buildImage(testContextTemplate{`
         from {IMAGE}

server/buildfile.go

@@ -338,7 +338,14 @@ func (b *buildFile) CmdCopy(args string) error {
 }
 
 func (b *buildFile) CmdWorkdir(workdir string) error {
+	if workdir[0] == '/' {
 		b.config.WorkingDir = workdir
+	} else {
+		if b.config.WorkingDir == "" {
+			b.config.WorkingDir = "/"
+		}
+		b.config.WorkingDir = filepath.Join(b.config.WorkingDir, workdir)
+	}
 	return b.commit("", b.config.Cmd, fmt.Sprintf("WORKDIR %v", workdir))
 }