Incorrect DNS options with host networking when buildkit is run as docker-container on a host with systemd-resolved

[lyoung-confluent]

[!NOTE]
This issue is closely related (possibly even a duplicate) to https://github.com/moby/buildkit/issues/2404 and https://github.com/moby/moby/pull/41022

When BuildKit is launched as a Docker container (either via the Docker container driver or via docker run and the remote driver) with "host" networking enabled (--network=host) on a host OS using systemd-resolved the incorrect DNS settings (/etc/resolv.conf) will be passed to build containers. Specifically, instead of using the host/system DNS settings it will fallback to the default configuration:

nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 2001:4860:4860::8888
nameserver 2001:4860:4860::8844

This issue occurs due to the use of resolveconf.Path which implements custom logic to detect the use of systemd-resolved: https://github.com/moby/moby/blob/de5c9cf0b96e4e172b96db54abababa4a328462f/libnetwork/internal/resolvconf/resolvconf_path.go#L50-L51

If it detects systemd-resolved it attempts to instead read/use the file at /run/systemd/resolve/resolv.conf, however this file will not exist within the running Docker container as it is running within a mount namespace. As a result, BuildKit will fallback to the default DNS servers (Google) which may not be reachable in the environment.

As a work-around, the host resolv.conf files can be mounted into the running buildkit container which will result in a functional build environment:

  --network host \
  --mount 'type=bind,readonly,src=/etc/resolv.conf,dst=/etc/resolv.conf' \
  --mount 'type=bind,readonly,src=/run/systemd/resolve/resolv.conf,dst=/run/systemd/resolve/resolv.conf' \

[thaJeztah]

cc @robmry