(renamed from: Remove use custom network for GitLab runners and buildkitd in favor of a fixed IP; see update below)
See T308271#8225621 for background as to why this is needed. The tl;dr is that buildkitd needs to know our host's resolv.conf configuration in order to pass it along to containers that it spawns, yet buildkitd is currently running within a Docker container that uses a custom Docker network which overwrites resolv.conf with an internal Docker DNS proxy.
The custom Docker network is currently serving only one purpose which is to provide a way to consistently reference the buildkitd server (by name). To retain this consistency, we'll assign a fixed IP to the buildkitd container within the default Docker network and set a runner level environment variable for BUILDKIT_HOST that obviates hardcoding of the IP in CI jobs.
update:
Unfortunately a fixed IP cannot be allocated to containers on the default network, so we can't abandon the custom network. However, there appears to be an undocumented config field for buildkitd to configure DNS options. This will allow us to pass along DNS options which buildkitd will use when spawning build containers. \o/