How do I enable custom network in build run with docker internal DNS?

[croepha]

So, lets say I have a service running in a container, and I want to use that the build of an image

Naively, I would do this:

docker network create net0
docker run -d --network net0 --name web0 nginx:latest # The service...
docker buildx create --driver=docker-container --driver-opt network=net0 --name buildkit0
docker buildx build --builder buildkit0 - << EOF                                                                                                                                                                                                                
FROM ubuntu:latest                                                                                                                                                                                                                                                                                             
RUN apt update && apt install -y dnsutils                                                                                                                                                                                                                                                                      
RUN cat /etc/resolv.conf && nslookup web0                                                                                                                                                                                                                                                                    
EOF

However, for some reason it is using a default resolve config, and wont use the internal dns server and cannot resolve web0:

 > [3/3] RUN cat /etc/resolv.conf && nslookup web0:                                                                                                                                                                                                                                                            
0.126 nameserver 8.8.8.8                                                                                                                                                                                                                                                                                       
0.126 nameserver 8.8.4.4                                                                                                                                                                                                                                                                                       
0.126 nameserver 2001:4860:4860::8888                                                                                                                                                                                                                                                                          
0.126 nameserver 2001:4860:4860::8844                                                                                                                                                                                                                                                                          
0.126 search .
0.126 options edns0 trust-ad ndots:0
0.179 Server:           8.8.8.8
0.179 Address:  8.8.8.8#53
0.179 
0.179 ** server can't find web0: NXDOMAIN

However if, I manually resolve the the internal dns server then it works:

 > [3/3] RUN nslookup web0 -- 127.0.0.11 ; false:                                                                                                                                                                                                                                                              
0.091 *** Invalid option: -                                                                                                                                                                                                                                                                                    
0.091 Server:           127.0.0.11                                                                                                                                                                                                                                                                             
0.091 Address:  127.0.0.11#53                                                                                                                                                                                                                                                                                  
0.091 
0.091 Non-authoritative answer:
0.091 Name:     web0
0.091 Address: 172.27.0.3

I suspect that the bad resolve config is coming from this code:

buildkit/executor/oci/resolvconf.go

Lines 108 to 114 in 7bf2360

	if netMode != pb.NetMode_HOST || len(resolvconf.GetNameservers(dt, resolvconf.IP)) == 0 {
	f, err := resolvconf.FilterResolvDNS(dt, true)
	if err != nil {
	return struct{}{}, errors.WithStack(err)
	}
	dt = f.Content
	}

which I think I can disable with host networking, but I'm not trying to do host networking, I'm trying to use a specific network

[croepha]

So adding --allow network.host to the buildx command line and --network=host to the RUN instruction in the Dockerfile seem to work, but this feels like I'm enabling a new level of access, when I already had the access without these command, I just didn't have the right text in my resolv.conf...