add process driver about buildx HOT 2 OPEN

Pidfile should be used for keeping track of the daemon and monitoring its state.

Process management can be tricky, perhaps docker-container driver does a pretty good job at this!

Perhaps this kind of driver might be more feasible for a setting where the buildkitd runs in foreground for a single build and expected to exit right away?

from buildx.

I thought a bit more about this driver, quick notes:

• Check buildkitd in path
  • If not available we can download latest buildkit release and extract to ~/.docker/buildx/.buildkit/<version> (use GitHub API to retrieve artifacts)
    • Also be able to set buildkit version through driver-opt
• PID in ~/.docker/buildx/.buildkit/<builder_name>/buildkitd.pid
• Define default BuildKit Daemon configuration for this driver with defaults (can be default buildkitd-flags too):

root = "~/.docker/buildx/.buildkit/<builder_name>/state"
insecure-entitlements = [ "network.host", "security.insecure" ] # not sure

[worker.oci]
  enabled = true
  snapshotter = "auto" # overlayfs or native, default value is "auto".
  rootless = true # rootless should be enabled by default (or disabled if "docker buildx create" run as root)
  binary = "~/.docker/buildx/.buildkit/<version>/buildkit-runc"

from buildx.