Add support for .dockerignore (or similar mechanism)

[tristanpemble]

I started to look into adding support for .dockerignore files when creating the build context, but I'm afraid I have no idea what I am doing. Seems like you can pull most of what you need from the standard Buildkit builder

Right now copying the build context takes quite a while due to my repository with a multi-gigabyte Git history

[AkihiroSuda]

Yes, we can just take this part from the Dockerfile frontend
https://github.com/moby/buildkit/blob/a1cfefeaeb66501a95a4d2f5858c939211f331ac/frontend/dockerfile/builder/build.go#L895-L910