Add user namespace (mapping) support to the Docker engine

Adds support for the daemon to handle user namespace maps as a
per-daemon setting.

Support for handling uid/gid mapping is added to the builder,
archive/unarchive packages and functions, all graphdrivers (except
Windows), and the test suite is updated to handle user namespace daemon
rootgraph changes.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

archive/changes.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/Sirupsen/logrus"
+	"github.com/docker/docker/pkg/idtools"
 	"github.com/docker/docker/pkg/pools"
 	"github.com/docker/docker/pkg/system"
 )
@@ -341,13 +342,15 @@ func ChangesSize(newDir string, changes []Change) int64 {
 }
 
 // ExportChanges produces an Archive from the provided changes, relative to dir.
-func ExportChanges(dir string, changes []Change) (Archive, error) {
+func ExportChanges(dir string, changes []Change, uidMaps, gidMaps []idtools.IDMap) (Archive, error) {
 	reader, writer := io.Pipe()
 	go func() {
 		ta := &tarAppender{
 			TarWriter: tar.NewWriter(writer),
 			Buffer:    pools.BufioWriter32KPool.Get(nil),
 			SeenFiles: make(map[uint64]string),
+			UIDMaps:   uidMaps,
+			GIDMaps:   gidMaps,
 		}
 		// this buffer is needed for the duration of this piped stream
 		defer pools.BufioWriter32KPool.Put(ta.Buffer)