Fix AttachToContainer bug

There's some funny behavior when you "attach" to a container but don't define the stdin stream.  Example:
```
#!/bin/bash -e
echo this goes to stdout
echo this goes to stderr >&2
```
outputs the following:
```
$ docker run -v /vagrant/code/mesa:/mesa  mesa:plume-build /mesa/test.sh
this goes to stderr
$
```

This corrects the issue