add binary target to direct install in a container

This add `make binary` to perform direct install libcontainer binaries inside
dockercore/libcontainer container instead of having to install dependencies and
run `make direct-install` on the host.

The `nsinit` binary will be placed into `bundles/nsinit`, and we ignore the
bundles directory from git.

Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com>

.gitignore

@@ -1 +1,2 @@
+bundles
 nsinit/nsinit

Makefile

@@ -29,3 +29,5 @@ local:
 validate:
 	hack/validate.sh
 
+binary: all
+	docker run --rm --privileged -v $(CURDIR)/bundles:/go/bin dockercore/libcontainer make direct-install

README.md

@@ -141,6 +141,9 @@ container.Resume()
 It is able to spawn new containers or join existing containers.  A root
 filesystem must be provided for use along with a container configuration file.
 
+To build `nsinit`, run `make binary`. It will save the binary into
+`bundles/nsinit`.
+
 To use `nsinit`, cd into a Linux rootfs and copy a `container.json` file into 
 the directory with your specified configuration. Environment, networking, 
 and different capabilities for the container are specified in this file.