Speed up Docker builds by using cache, mounted from Docker host

Psst ... this is useful across many package managers.
Personally, I've used it and seen it working with pip, go and apt.
It must be there in other package managers as well (it'd be really dumb if they're downloading what they already have).

References:

Github Repo:

[maxde]

Well done. I'm working with docker for many years now, and I didn't knew about this! Earned this sub.