@@ -247,46 +247,10 @@ func runBuild(dockerCli *command.DockerCli, options buildOptions) error {

 	// replace Dockerfile if added dynamically

 	if dockerfileCtx != nil {

-		file, err := ioutil.ReadAll(dockerfileCtx)

-		dockerfileCtx.Close()

+		buildCtx, relDockerfile, err = addDockerfileToBuildContext(dockerfileCtx, buildCtx)

 		if err != nil {

 			return err

 		}

-		now := time.Now()

-		hdrTmpl := &tar.Header{

-			Mode:       0600,

-			Uid:        0,

-			Gid:        0,

-			ModTime:    now,

-			Typeflag:   tar.TypeReg,

-			AccessTime: now,

-			ChangeTime: now,

-		}

-		randomName := ".dockerfile." + stringid.GenerateRandomID()[:20]

-

-		buildCtx = archive.ReplaceFileTarWrapper(buildCtx, map[string]archive.TarModifierFunc{

-			randomName: func(_ string, h *tar.Header, content io.Reader) (*tar.Header, []byte, error) {

-				return hdrTmpl, file, nil

-			},

-			".dockerignore": func(_ string, h *tar.Header, content io.Reader) (*tar.Header, []byte, error) {

-				if h == nil {

-					h = hdrTmpl

-				}

-				extraIgnore := randomName + "\n"

-				b := &bytes.Buffer{}

-				if content != nil {

-					_, err := b.ReadFrom(content)

-					if err != nil {

-						return nil, nil, err

-					}

-				} else {

-					extraIgnore += ".dockerignore\n"

-				}

-				b.Write([]byte("\n" + extraIgnore))

-				return h, b.Bytes(), nil

-			},

-		})

-		relDockerfile = randomName

 	}

 	ctx := context.Background()

@@ -392,6 +356,51 @@ func runBuild(dockerCli *command.DockerCli, options buildOptions) error {

 	return nil

 }

+func addDockerfileToBuildContext(dockerfileCtx io.ReadCloser, buildCtx io.ReadCloser) (io.ReadCloser, string, error) {

+	file, err := ioutil.ReadAll(dockerfileCtx)

+	dockerfileCtx.Close()

+	if err != nil {

+		return nil, "", err

+	}

+	now := time.Now()

+	hdrTmpl := &tar.Header{

+		Mode:       0600,

+		Uid:        0,

+		Gid:        0,

+		ModTime:    now,

+		Typeflag:   tar.TypeReg,

+		AccessTime: now,

+		ChangeTime: now,

+	}

+	randomName := ".dockerfile." + stringid.GenerateRandomID()[:20]

+

+	buildCtx = archive.ReplaceFileTarWrapper(buildCtx, map[string]archive.TarModifierFunc{

+		// Add the dockerfile with a random filename

+		randomName: func(_ string, h *tar.Header, content io.Reader) (*tar.Header, []byte, error) {

+			return hdrTmpl, file, nil

+		},

+		// Update .dockerignore to include the random filename

+		".dockerignore": func(_ string, h *tar.Header, content io.Reader) (*tar.Header, []byte, error) {

+			if h == nil {

+				h = hdrTmpl

+			}

+			extraIgnore := randomName + "\n"

+			b := &bytes.Buffer{}

+			if content != nil {

+				_, err := b.ReadFrom(content)

+				if err != nil {

+					return nil, nil, err

+				}

+			} else {

+				extraIgnore += ".dockerignore\n"

+			}

+			b.Write([]byte("\n" + extraIgnore))

+			return h, b.Bytes(), nil

+		},

+	})

+	return buildCtx, randomName, nil

+}

+

 func isLocalDir(c string) bool {

 	_, err := os.Stat(c)

 	return err == nil

@@ -230,10 +230,8 @@ func CompressStream(dest io.Writer, compression Compression) (io.WriteCloser, er

 // define a modification step for a single path

 type TarModifierFunc func(path string, header *tar.Header, content io.Reader) (*tar.Header, []byte, error)

-// ReplaceFileTarWrapper converts inputTarStream to a new tar stream

-// while replacing a single file called header.Name with new contents.

-// If the file with header.Name does not exist it is added to the tar stream.

-// TODO: make this into a generic tar conversion function with walkFn argument

+// ReplaceFileTarWrapper converts inputTarStream to a new tar stream. Files in the

+// tar stream are modified if they match any of the keys in mods.

 func ReplaceFileTarWrapper(inputTarStream io.ReadCloser, mods map[string]TarModifierFunc) io.ReadCloser {

 	pipeReader, pipeWriter := io.Pipe()

@@ -255,7 +253,7 @@ func ReplaceFileTarWrapper(inputTarStream io.ReadCloser, mods map[string]TarModi

 			for len(modKeys) > 0 && (err == io.EOF || err == nil && hdr.Name >= modKeys[0]) {

 				var h *tar.Header

 				var rdr io.Reader

-				if hdr != nil && hdr.Name == modKeys[0] {

+				if err == nil && hdr != nil && hdr.Name == modKeys[0] {

 					h = hdr

 					rdr = tarReader

 				}