• related DOCKER_BUILDKIT=1 prevents custom networks using docker build --network moby/buildkit#978

Background:
Running a simple integration test fails with network option:

docker network create custom_network
docker run -d --network custom_network --name mongo mongo:3.6
docker buildx build --network custom_network --target=test .

Output:
network mode "custom_network" not supported by buildkit

Still not supported? Code related:
https://github.com/docker/buildx/blob/master/build/build.go#L462-L463