`httpProxy` support

I'm running docker in a network with http proxy which is configured to be used in docker using proxies.default.httpProxy configuration directive.

It works fine with the default docker.

When I enable buildkit the networking operations fails with

 > [2/2] RUN apt update && apt install -y --no-install-recommends zip:
#7 0.383 
#7 0.383 WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
#7 0.383 
#7 0.657 Err:1 http://security.ubuntu.com/ubuntu bionic-security InRelease
#7 0.657   Could not resolve 'cache.org.tld'
#7 0.913 Err:2 http://archive.ubuntu.com/ubuntu bionic InRelease
#7 0.913   Could not resolve 'cache.org.tld'
#7 0.913 Err:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
#7 0.913   Could not resolve 'cache.org.tld'
#7 0.913 Err:4 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
#7 0.913   Could not resolve 'cache.org.tld'
#7 0.921 Reading package lists...

cache.org.tld' (redacted) is a valid hostname, resolvable by LAN DNS.

I tried to specify dns servers additionally in the /etc/docker/daemon.json, it does not change the outcome though.

A Dockerfile for the output above:

# syntax = docker/dockerfile:1.0-experimental
FROM ubuntu:bionic as base

RUN apt update && apt install -y --no-install-recommends zip

Is it

Start with the provided Dockerfile and the interaction between BuildKit and Docker's proxies.default.httpProxy setting. Reproduce the failed apt update with the shown Ubuntu image, compare it with the daemon.json DNS configuration, and define done as networking through the configured HTTP proxy working when BuildKit is enabled.