Enabling userns_host

[neonihil]

Buildkit bootstrap is failing when docker user namespace mapping is enabled:

failed to create the builder: failed to create buildkit client: failed to bootstrap the buildkitd: failed to create container: Error response from daemon: privileged mode is incompatible with user namespaces.  You must run the container in the host namespace when running privileged mode

This PR is passing --userns host to the docker startup command to prevent this.