Enable BuildKit (experimental) on Pipelines

https://github.com/moby/buildkit

My use case:

At the moment I run docker-compose build within BitBucket Pipelines to build .NET Core code. Pipelines has a NuGet cache, but I can't use this because Docker builds run within a container on Docker, and you can't mount volumes into builds with Docker.

I spoke to one of the maintainers for docker-compose and I've been informed that BuildKit allows this via the cache mounting type which would allow me to use it within my pipelines builds.

Admittedly there is an alternative in the form of moving my build directly into pipelines, but this would prevent me reproducing the exact same build locally, etc.