Fix pip upgrade command in Dockerfile

[ychikazawa]

Summary

Now pip version must be 21.0 or lower in Python 2.7 environment.
pip install --upgrade "pip < 21.0"

Details

Python 2.7 is no longer supported since pip 21.0.
https://pip.pypa.io/en/stable/news/#v20-1
https://stackoverflow.com/questions/65869296/installing-pip-is-not-working-in-python-3-6/65871131#65871131

But pip install --upgrade pip tried to install pip version 21.0 or higher even with python2.7 and gave the following error.

#7 7.735 Collecting pip
#7 7.846   Downloading https://files.pythonhosted.org/packages/4d/0c/3b63fe024414a8a48661cf04f0993d4b2b8ef92daed45636474c018cd5b7/pip-21.1.3.tar.gz (1.6MB)
#7 8.543 Building wheels for collected packages: pip
#7 8.544   Running setup.py bdist_wheel for pip: started
#7 8.697   Running setup.py bdist_wheel for pip: finished with status 'error'
#7 8.697   Complete output from command /usr/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-IikqTW/pip/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" bdist_wheel -d /tmp/tmp3VYWDQpip-wheel- --python-tag cp27:
#7 8.697   /usr/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'project_urls'
#7 8.697     warnings.warn(msg)
#7 8.697   /usr/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'python_requires'
#7 8.697     warnings.warn(msg)
#7 8.697   usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
#7 8.697      or: -c --help [cmd1 cmd2 ...]
#7 8.697      or: -c --help-commands
#7 8.697      or: -c cmd --help
#7 8.697
#7 8.697   error: invalid command 'bdist_wheel'
#7 8.697
#7 8.697   ----------------------------------------
#7 8.698   Failed building wheel for pip
#7 8.698   Running setup.py clean for pip
#7 8.848 Failed to build pip
#7 8.848 Installing collected packages: pip
#7 8.849   Found existing installation: pip 8.1.1
#7 8.881     Not uninstalling pip at /usr/lib/python2.7/dist-packages, outside environment /usr
#7 8.882   Running setup.py install for pip: started
#7 9.831     Running setup.py install for pip: finished with status 'done'
#7 9.890 Successfully installed pip-21.1.3

[arcturusannamalai]

python2.7 support may not be general interest since its retired by Python community.
Perhaps this change should only go in some backwards maintainable port of Caffe.