Parallelize pull_images command

[mschwager]

I noticed that running pull_images takes quite a while on first run. This PR parallelizes the pull_images command in the same way as download_corpora.

[jonathanmetzman]

I noticed that running pull_images takes quite a while on first run. This PR parallelizes the pull_images command in the same way as download_corpora.

Can you do a test on how much this improves things? And maybe hide this behind a flag?

I did (what I think, but is unmeasured) slightly better version of this for pushing, that takes into account dependencies (e.g. don't push base-image and base-clang at the same time, since base-clang inherits from base-image.

[mschwager]

Hey! Sorry, I won't have time to look back into this for at least another couple months.

The runtime improvement wasn't too great because docker pull mostly saturates the network anyway. There might be some slight improvements when one docker pull thread is unpacking an already downloaded image and another thread is actively downloading an image. It was definitely a bit quick on my machine, but maybe 10-20%, nothing substantial. And this is for an infrequently run command.

Either way, feel free to close this if it's not worth it! 👍