Fix `docker exec -u` issue after docker daemon restart

This fix tries to address the issue raised in 29342 where
`docker exec -u` after docker daemon restart returns an error:
```
unable to find user test: no matching entries in passwd file
```

The reason was that `container.BaseFS` is not present after restart.

This fix adds the `daemon.Mount` during the restore to bring up the
`container.BaseFS`.

An integration test has been added to cover the changes.

This fix fixes 29342.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
(cherry picked from commit 7feb2a17e4b9d1a5305a8a44004e916b79cbdd97)
Signed-off-by: Victor Vieux <vieux@docker.com>

daemon/daemon.go

@@ -149,6 +149,10 @@ func (daemon *Daemon) restore() error {
 				continue
 			}
 			container.RWLayer = rwlayer
+			if err := daemon.Mount(container); err != nil {
+				logrus.Errorf("Failed to mount container %v: %v", id, err)
+				continue
+			}
 			logrus.Debugf("Loaded container %v", container.ID)
 
 			containers[container.ID] = container

integration-cli/docker_cli_daemon_test.go

@@ -2927,3 +2927,28 @@ func (s *DockerDaemonSuite) TestDaemonShutdownTimeoutWithConfigFile(c *check.C)
 	content, _ := ioutil.ReadFile(s.d.logFile.Name())
 	c.Assert(string(content), checker.Contains, expectedMessage)
 }
+
+// Test case for 29342
+func (s *DockerDaemonSuite) TestExecWithUserAfterLiveRestore(c *check.C) {
+	testRequires(c, DaemonIsLinux)
+	s.d.StartWithBusybox("--live-restore")
+
+	out, err := s.d.Cmd("run", "-d", "--name=top", "busybox", "sh", "-c", "addgroup -S test && adduser -S -G test test -D -s /bin/sh && top")
+	c.Assert(err, check.IsNil, check.Commentf("Output: %s", out))
+
+	waitRun("top")
+
+	out1, err := s.d.Cmd("exec", "-u", "test", "top", "id")
+	// uid=100(test) gid=101(test) groups=101(test)
+	c.Assert(err, check.IsNil, check.Commentf("Output: %s", out1))
+
+	// restart daemon.
+	s.d.Restart("--live-restore")
+
+	out2, err := s.d.Cmd("exec", "-u", "test", "top", "id")
+	c.Assert(err, check.IsNil, check.Commentf("Output: %s", out2))
+	c.Assert(out1, check.Equals, out2, check.Commentf("Output: before restart '%s', after restart '%s'", out1, out2))
+
+	out, err = s.d.Cmd("stop", "top")
+	c.Assert(err, check.IsNil, check.Commentf("Output: %s", out))
+}