When running linuxkit behind a transparent caching proxy that uses DNS to reroute requsts to *.docker.io to NGINX proxy we need a way to inject NGINX'es certificate into linuxkit-builder container to trust it. both docker builder create and buildx builder commands allow to do so so it should be fairly simple to implement.

1. The buldkit documentation describes certificate injection https://docs.docker.com/build/buildkit/configure/#setting-registry-certificates

# /etc/buildkitd.toml
debug = true
[registry."myregistry.com"]
  ca=["/etc/certs/myregistry.pem"]
  [[registry."myregistry.com".keypair]]
    key="/etc/certs/myregistry_key.pem"
    cert="/etc/certs/myregistry_cert.pem"

2. Linuxkit already has an option to provide a custom config for builder
3. we just missing an option to mount certificates to buildkit container.