Dockerfile: retain the pip download cache between builds

[SpecLad]

Motivation and context

This speeds up the build when the entire step can't be cached (e.g. the requirements file changed), but the package list remains mostly the same.

The savings are... rather underwhelming, actually. I have observed about a minute in savings, although it obviously depends on the network connection speed. I think this is because pip is inefficient at loading from its own cache (I have observed it loading the entire cached file into memory, for example).

Still, savings are savings, and we're getting them basically for free, so why not.

Note that I only persist the HTTP cache, and not the wheel cache. That's because any wheels that pip builds could depend on the system packages, and I don't want old wheels to be reused if the system packages change.

Also, disable the pip autoupdate checks, which isn't much of an optimization, but it gets rid of some pointless warnings.

How has this been tested?

docker build

Checklist

• I submit my changes into the develop branch
• [ ] I have added a description of my changes into the CHANGELOG file
• [ ] I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [ ] I have linked related issues (see GitHub docs)
• [ ] I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.