Generate intermediate image into a temp file

All of the `output*` functions took a `[]byte` and immediately wrapped it in a
`bytes.Buffer` to produce an `io.Reader`. Make them take an `io.Reader` instead
and satisfy this further up the call chain by directing `moby.Build` to output
to a temp file instead of another `bytes.Buffer`.

In my test case (building kube master image) this reduces Maximum RSS (as
measured by time(1)) from 6.7G to 2.8G and overall allocations from 9.7G to
5.3G. When building a tar (output to /dev/null) the Maximum RSS fell slightly
from 2.2G to 2.1G. Overall allocations remained stable at around 5.3G.

Signed-off-by: Ian Campbell <ijc@docker.com>

src/moby/linuxkit.go

@@ -1,7 +1,6 @@
 package moby
 
 import (
-	"bytes"
 	"crypto/sha256"
 	"fmt"
 	"io"
@@ -58,9 +57,21 @@ func ensureLinuxkitImage(name string) error {
 		return err
 	}
 	// TODO pass through --pull to here
-	buf := new(bytes.Buffer)
-	Build(m, buf, false, "")
-	image := buf.Bytes()
+	tf, err := ioutil.TempFile("", "")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(tf.Name())
+	Build(m, tf, false, "")
+	if err := tf.Close(); err != nil {
+		return err
+	}
+
+	image, err := os.Open(tf.Name())
+	if err != nil {
+		return err
+	}
+	defer image.Close()
 	kernel, initrd, cmdline, err := tarToInitrd(image)
 	if err != nil {
 		return fmt.Errorf("Error converting to initrd: %v", err)