cleaning unnecessary pip cache

[aminvakil]

https://pip.pypa.io/en/stable/reference/pip_install/#caching

It's almost 30MB difference in a 3GB docker image:)
But it's safe to use and I can't think of a situation where pip cache in creating this image is needed.

[johndpope]

While this will help the pip cache - it will invalidate the image for stylegan2 in dockers' build cache
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#leverage-build-cache

this means in practical terms - many users will need to refetch / rebuild gigabytes of data. It adds up if / when you have more than one image using stylegan2 base dockerfile.

[aminvakil]

Just to clarify what does my commit do:

pip install --no-cache-dir is suitable in two situations:

1. When you want to be sure you're installing the latest version of the package from pip repo. By default if the package you want to install has been installed before and it hasn't expired yet (From pip documentation: When storing items in the cache, pip will respect the CacheControl header if it exists, or it will fall back to the Expires header if that exists. This allows pip to function as a browser would, and allows the index server to communicate to pip how long it is reasonable to cache any particular item.), pip will install the downloaded package from cache.

2. When you don't want to cache the packages you're installing, this is the scenario which is meant here in this commit. We don't want our image to keep pip cache, because it isn't needed later.

This has nothing to do with docker build cache that you have said, docker doesn't care what you're doing in each RUN in your Dockerfile, it will build another layer after each command that you've written in your Dockerfile, whether it's just an ls or it's pip install package or pip install --no-cache-dir package, the only difference here is that the layer which docker creates with the committed Dockerfile is smaller than the main Dockerfile.