Update Dockerfile: set pip version below 21.0 due to drop support of python 2

[justinchuch]

After pip version is updated to 21.0 or up, docker/build.sh -t -i -n would have the following error:

Step 19/28 : RUN pip install virtualenv virtualenvwrapper
 ---> Running in 543385b5721a
Traceback (most recent call last):
  File "/usr/local/bin/pip", line 7, in <module>
    from pip._internal.cli.main import main
  File "/usr/local/lib/python2.7/dist-packages/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax
The command '/bin/sh -c pip install virtualenv virtualenvwrapper' returned a non-zero code: 1

This is due to pip dropping support of python 2 starting from version 21.0

https://pip.pypa.io/en/stable/news/#id4

pypa/pip#6148

[spmallette]

Thank you - merged.