Upgrade both docker and buildx

[jeromegn]

buildx uses a much more recent buildkit which hopefully fixes a few bugs we've encountered.

[jsierles]

FWIW, I had done this before and some build started crashing. However it could have been a coincidence, or related to the Ubuntu base images. Worth a try again!

[jeromegn]

@jsierles oh, good to know. I'll try it out manually first.

[jsierles]

Cool. You can use fly orgs builder to set it on an org (in case you didn't know about that command).

[jeromegn]

Cool. You can use fly orgs builder to set it on an org (in case you didn't know about that command).

I did not know! Interesting.