Add docs for reading Dockerfile from stdin.

Signed-off-by: Daniel Nephin <dnephin@docker.com>
Upstream-commit: e838679cd7954775cbe7ff72ef2da02dbe791fcb
Component: engine

components/engine/docs/reference/commandline/build.md

@@ -344,6 +344,13 @@ $ docker build -f Dockerfile.debug .
 This will use a file called `Dockerfile.debug` for the build instructions
 instead of `Dockerfile`.
 
+```bash
+$ curl example.com/remote/Dockerfile | docker build -f - .
+```
+
+The above command will use the current directory as the build context and read
+a Dockerfile from stdin.
+
 ```bash
 $ docker build -f dockerfiles/Dockerfile.debug -t myapp_debug .
 $ docker build -f dockerfiles/Dockerfile.prod  -t myapp_prod .

components/engine/pkg/archive/archive_test.go

@@ -4,7 +4,6 @@ import (
 	"archive/tar"
 	"bytes"
 	"fmt"
-	"github.com/docker/docker/pkg/testutil/assert"
 	"io"
 	"io/ioutil"
 	"os"
@@ -14,6 +13,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/docker/docker/pkg/testutil/assert"
 )
 
 var tmp string