Specify numpy==1.18.5 since latest default v1.19+ is incompatible

[rajitbanerjee]

@ExSidius The current latest docker image (https://hub.docker.com/r/exsidius/openpose) is broken since the 20-JUN-2020 release of numpy v1.19.0, which only supports Python 3.6+. The default python3 installation on Ubuntu 16.04 is v3.5.2, hence the specific numpy=1.18.5 supporting Python 3.5 needs to be mentioned in the Dockerfile.

The README.md file has also been updated to reflect the deprecation of nvidia-docker in the Docker versions 19.03+. The build steps mentioned have been tested.

[suhrmann]

Instead of nailing Numpy version down I think updating pip before insallation might avoid further problems. Here's my solution:

28	. . .   && \
29	python3 -m pip install --upgrade pip && \
30	python3 -m pip install \
31	numpy \
32	protobuf \
33	opencv-python