Agree. Using a container to build the source that is then packaged as a "binary" in the resulting container always seemed odd to me. imho we should have stuck with the old ways : build the product on a regular computer. That outputs some build artifacts (binaries, libraries, etc). Docker should take those artifacts and not be hosting the compiler and what not.
nix allows you to build docker containers with anything you can build in nix.
If anything the build being in a container is the more valuable bit, though mainly because the container usually more repeatable by having a scripted setup itself. Though I dunno why the build and the host would be the _same_ container in the end.
(and of course, nix kinda blows both out the water for consistency)