Docker should use /var/lib/container/tmp for large temporary files.

/tmp is often a tmpfs file system and large temporary files could cause
docker commands to fail.  Also using /tmp potentially allows users on the
system to get access to content, or even attack the content.  Moving the tmpdir to
/var/lib/container/tmp will protect the data.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

Conflicts:
	docker/docker.go

utils/tmpdir_unix.go

@@ -0,0 +1,18 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package utils
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// TempDir returns the default directory to use for temporary files.
+func TempDir(rootDir string) (string, error) {
+	var tmpDir string
+	if tmpDir = os.Getenv("DOCKER_TMPDIR"); tmpDir == "" {
+		tmpDir = filepath.Join(rootDir, "tmp")
+	}
+	err := os.MkdirAll(tmpDir, 0700)
+	return tmpDir, err
+}