Set OS on scratch image and prevent panic if empty

Signed-off-by: John Stephens <johnstep@docker.com>

builder/dockerfile/imagecontext.go

@@ -1,6 +1,8 @@
 package dockerfile
 
 import (
+	"runtime"
+
 	"github.com/docker/docker/api/types/backend"
 	"github.com/docker/docker/builder"
 	"github.com/docker/docker/builder/remotecontext"
@@ -73,7 +75,13 @@ func (m *imageSources) Unmount() (retErr error) {
 func (m *imageSources) Add(im *imageMount) {
 	switch im.image {
 	case nil:
-		im.image = &dockerimage.Image{}
+		// set the OS for scratch images
+		os := runtime.GOOS
+		// Windows does not support scratch except for LCOW
+		if runtime.GOOS == "windows" {
+			os = "linux"
+		}
+		im.image = &dockerimage.Image{V1Image: dockerimage.V1Image{OS: os}}
 	default:
 		m.byImageID[im.image.ImageID()] = im
 	}

daemon/create.go

@@ -95,7 +95,14 @@ func (daemon *Daemon) create(params types.ContainerCreateConfig, managed bool) (
 		if err != nil {
 			return nil, err
 		}
+		if img.OS != "" {
 			os = img.OS
+		} else {
+			// default to the host OS except on Windows with LCOW
+			if runtime.GOOS == "windows" && system.LCOWSupported() {
+				os = "linux"
+			}
+		}
 		imgID = img.ID()
 
 		if runtime.GOOS == "windows" && img.OS == "linux" && !system.LCOWSupported() {

integration-cli/docker_api_build_test.go

@@ -619,6 +619,28 @@ func testBuildWithSession(c *check.C, dir, dockerfile string) (outStr string) {
 	return
 }
 
+func (s *DockerSuite) TestBuildScratchCopy(c *check.C) {
+	testRequires(c, DaemonIsLinux)
+	dockerfile := `FROM scratch
+ADD Dockerfile /
+ENV foo bar`
+	ctx := fakecontext.New(c, "",
+		fakecontext.WithDockerfile(dockerfile),
+	)
+	defer ctx.Close()
+
+	res, body, err := request.Post(
+		"/build",
+		request.RawContent(ctx.AsTarReader(c)),
+		request.ContentType("application/x-tar"))
+	c.Assert(err, checker.IsNil)
+	c.Assert(res.StatusCode, checker.Equals, http.StatusOK)
+
+	out, err := request.ReadBody(body)
+	require.NoError(c, err)
+	assert.Contains(c, string(out), "Successfully built")
+}
+
 type buildLine struct {
 	Stream string
 	Aux    struct {