This adds the ability to source additional local build definition files when
sourcing Bake files via a remote url.
Prefixing a file with 'cwd://' will source a bake file on the local
machine, instead of the remote location.
Local files will be read/have precedence before remote files.
Usage:
```
docker buildx bake https://github.com/example/upstream.git --file cwd://docker-bake.override.hcl --print
```
This will source a default file from the example/upstream repository,
and also source a build definition from the local machine.
Signed-off-by: Cameron Adams <pnzreba@gmail.com>
78616f2531