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>
Fixed in 12b6a3ad9a, but now we have
regression tests! So we can add a check that we don't break this
behavior again.
Signed-off-by: Justin Chadwell <me@jedevc.com>
Cameron Adams