WITH DOCKER --cache-id needs locking on the buildkit-side

[alexcb]

What went wrong?

#3399 was completed without thinking of the case where two earthly processes (possibly on different hosts)

What should have happened?

we need to acquire a lock on the buildkit host; This can possibly be done with a file lock (flock) in https://github.com/earthly/earthly/blob/main/buildkitd/dockerd-wrapper.sh

What earthly version?

all of them

Buildkit Logs

No response

Other Helpful Information

No response