Reopening stdin, stdout and stderr if they are pointing to /dev/null to not have '/dev/null' from the global namespace opened inside the container.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #107 from vishh/dev_null

Reopening stdin, stdout and stderr if they are pointing to /dev/null.