I noticed that when I use the BAKE_CMD_CONTEXT built-in variable in a local HCL bake file, the build fails.

Steps to reproduce:

1. Create a Dockerfile (named "Dockerfile"):

FROM alpine
CMD ["sleep", "infinity"]

2. Create a bake file (named "docker-bake.hcl"):

target "default" {
  dockerfile = "Dockerfile"
  context = BAKE_CMD_CONTEXT
  tags = ["myimage:myversion"]
}

3. Run docker buildx bake

Expected outcome:
The docker image specified should be built

Actual outcome:

ERROR: could not find cwd:: stat cwd:: no such file or directory

I looked into it a bit further, and it seems that when the aforementioned built-in variable was introduced, while the context was arranged to be "cleaned" from the cwd protocol, the dockerfilePath is left with a cwd reference in it. Perhaps something as simple as adding a !strings.HasPrefix(contextPath, "cwd://") check here would be sufficient but take that with a pinch of salt as I'm not all that familiar with the codebase.

Also, if this variable was never meant to work in a local setting, please close this issue.