support for buildkit

[kennyworkman]

use of buildkit to build containers is desirable because:

• use of heredocs
• cache mounts
• build speed

buildkit is not supported out of the box in py-docker - docker/docker-py#2230

closing this issue means either patching py-docker ourself or waiting for it to get patched

[mnemeth66]

this looked like it would be a lot of work for not enough reward when I went down the rabbit hole, unless all images are being built by the same docker daemon in remote register

[kennyworkman]

the caching improvements will be quite nice when the cache is big - with improvements to remote builds, that's certainly the direction we're moving towards

additionally, the heredoc syntax is far more legible for multi-line RUN commands

shame...

[nh13]

Yes please!