When installing with pip, downloads are cached in ~/.pip/cache, theoretically to save time when installing multiple times. However, with docker this is useless because

• the home directory is not shared between instances of docker build, so the cache isn't used
• storing data in the cache dir makes the docker image larger than it needs to be, increasing time needed to push/pull

To prevent pip from caching files on installation, use the --no-cache-dir argument