Updated Dockerfile and resolved torch/np.bool related error.

[montioo]

Dockerfile now works and has additional code to optionally install ROS and OpenCV as a lot of users who wanted to get maskrcnn-benchmark to run, seem to use it in the context of an already existing project.
Furthermore, some errors related to torch/np.bool types were resolved by casting the data to torch/np.uint8. This got inference and training working again for me.
The readme holds links to all changes where the solutions have been discussed in corresponding issues. There's also a link to a setup tutorial to get things up and running.

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