Update the vendor folder (#53)

* Update the vendor folder

* Update the Gopkg.lock by running dep ensure

vendor/github.com/opencontainers/runc/libcontainer/nsenter/README.md

@@ -0,0 +1,25 @@
+## nsenter
+
+The `nsenter` package registers a special init constructor that is called before 
+the Go runtime has a chance to boot.  This provides us the ability to `setns` on 
+existing namespaces and avoid the issues that the Go runtime has with multiple 
+threads.  This constructor will be called if this package is registered, 
+imported, in your go application.
+
+The `nsenter` package will `import "C"` and it uses [cgo](https://golang.org/cmd/cgo/)
+package. In cgo, if the import of "C" is immediately preceded by a comment, that comment, 
+called the preamble, is used as a header when compiling the C parts of the package.
+So every time we  import package `nsenter`, the C code function `nsexec()` would be 
+called. And package `nsenter` is now only imported in Docker execdriver, so every time 
+before we call `execdriver.Exec()`, that C code would run.
+
+`nsexec()` will first check the environment variable `_LIBCONTAINER_INITPID` 
+which will give the process of the container that should be joined. Namespaces fd will 
+be found from `/proc/[pid]/ns` and set by `setns` syscall.
+
+And then get the pipe number from `_LIBCONTAINER_INITPIPE`, error message could
+be transfered through it. If tty is added, `_LIBCONTAINER_CONSOLE_PATH` will 
+have value and start a console for output.
+
+Finally, `nsexec()` will clone a child process , exit the parent process and let 
+the Go runtime take over.