Fix apex installation path in Dockerfile

[akihironitta]

What does this PR do?

Fixes #11566 and unblocks #11567.

Problem

So, the problem is that apex is somehow installed to:

/usr/lib/python3.7/site-packages/  # which is not in `sys.path`

while other dependencies, for example torch, are installed to:

/usr/local/lib/python3.7/dist-packages/

Not sure why pip install the libraries in different directories...

Description of the change

To fix this, I just added --user so that it installs apex to:

/root/.local/lib/python3.7/site-packages/

which is part of sys.path:

$ python -m site
sys.path = [
    '/',
    '/usr/lib/python37.zip',
    '/usr/lib/python3.7',
    '/usr/lib/python3.7/lib-dynload',
    '/usr/local/lib/python3.7/dist-packages',
    '/usr/lib/python3/dist-packages',
]
USER_BASE: '/root/.local' (exists)
USER_SITE: '/root/.local/lib/python3.7/site-packages' (exists)
ENABLE_USER_SITE: True

Does your PR introduce any breaking changes? If yes, please list them.

None

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

It was fruitful to finally get my hands on docker :)

cc @Borda @tchaton @rohitgr7 @akihironitta @carmocca