docker build: accept -f - to read Dockerfile from stdin

Implements proposal moby#27393 that describes the meaning of '-f -' in a
`docker build` command. For all context types except reading a context from
stdin, '-f -' will replace any file called 'Dockerfile' in the context's
tar stream with the contents of stdin. If the context itself is read from
stdin, '-f -' continues to read the Dockerfile called '-' from this context.

Fixes moby#27393.

Signed-off-by: David Sheets <sheets@alum.mit.edu>

cli/command/image/build.go

@@ -6,10 +6,12 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"time"
 
 	"github.com/docker/distribution/reference"
 	"github.com/docker/docker/api"
@@ -167,6 +169,10 @@ func runBuild(dockerCli *command.DockerCli, options buildOptions) error {
 		return fmt.Errorf("unable to prepare context: path %q not found", specifiedContext)
 	}
 
+	if options.dockerfileName == "-" {
+		relDockerfile = builder.DefaultDockerfileName
+	}
+
 	if err != nil {
 		if options.quiet && urlutil.IsURL(specifiedContext) {
 			fmt.Fprintln(dockerCli.Err(), progBuff)
@@ -232,6 +238,28 @@ func runBuild(dockerCli *command.DockerCli, options buildOptions) error {
 		}
 	}
 
+	if options.dockerfileName == "-" && specifiedContext != "-" {
+		stdin := dockerCli.In()
+		file, err := ioutil.ReadAll(stdin)
+		stdin.Close()
+		if err != nil {
+			return err
+		}
+		now := time.Now()
+		fileHdr := tar.Header{
+			Name:       builder.DefaultDockerfileName,
+			Mode:       0600,
+			Uid:        0,
+			Gid:        0,
+			Size:       int64(len(file)),
+			ModTime:    now,
+			Typeflag:   tar.TypeReg,
+			AccessTime: now,
+			ChangeTime: now,
+		}
+		buildCtx = archive.ReplaceFileTarWrapper(buildCtx, fileHdr, file)
+	}
+
 	ctx := context.Background()
 
 	var resolvedTags []*resolvedTag

cli/command/image/build/context.go

@@ -165,7 +165,7 @@ func GetContextFromLocalDir(localDir, dockerfileName string) (absContextDir, rel
 	// When using a local context directory, when the Dockerfile is specified
 	// with the `-f/--file` option then it is considered relative to the
 	// current directory and not the context directory.
-	if dockerfileName != "" {
+	if dockerfileName != "" && dockerfileName != "-" {
 		if dockerfileName, err = filepath.Abs(dockerfileName); err != nil {
 			return "", "", fmt.Errorf("unable to get absolute path to Dockerfile: %v", err)
 		}
@@ -219,6 +219,8 @@ func getDockerfileRelPath(givenContextDir, givenDockerfile string) (absContextDi
 				absDockerfile = altPath
 			}
 		}
+	} else if absDockerfile == "-" {
+		absDockerfile = filepath.Join(absContextDir, DefaultDockerfileName)
 	}
 
 	// If not already an absolute path, the Dockerfile path should be joined to
@@ -233,18 +235,20 @@ func getDockerfileRelPath(givenContextDir, givenDockerfile string) (absContextDi
 	// an issue in golang. On Windows, EvalSymLinks does not work on UNC file
 	// paths (those starting with \\). This hack means that when using links
 	// on UNC paths, they will not be followed.
-	if !isUNC(absDockerfile) {
-		absDockerfile, err = filepath.EvalSymlinks(absDockerfile)
-		if err != nil {
-			return "", "", fmt.Errorf("unable to evaluate symlinks in Dockerfile path: %v", err)
+	if givenDockerfile != "-" {
+		if !isUNC(absDockerfile) {
+			absDockerfile, err = filepath.EvalSymlinks(absDockerfile)
+			if err != nil {
+				return "", "", fmt.Errorf("unable to evaluate symlinks in Dockerfile path: %v", err)
+			}
 		}
-	}
 
-	if _, err := os.Lstat(absDockerfile); err != nil {
-		if os.IsNotExist(err) {
-			return "", "", fmt.Errorf("Cannot locate Dockerfile: %q", absDockerfile)
+		if _, err := os.Lstat(absDockerfile); err != nil {
+			if os.IsNotExist(err) {
+				return "", "", fmt.Errorf("Cannot locate Dockerfile: %q", absDockerfile)
+			}
+			return "", "", fmt.Errorf("unable to stat Dockerfile: %v", err)
 		}
-		return "", "", fmt.Errorf("unable to stat Dockerfile: %v", err)
 	}
 
 	if relDockerfile, err = filepath.Rel(absContextDir, absDockerfile); err != nil {