Potentially - if Nix or Bazel has already built the binaries, and just needs to construct an OCI-compliant image tarball with them, that can be quite quick, similar to a Dockerfile with only COPY instructions. Nix and Bazel can also give you deterministic / reproducible images that take more effort to construct with Dockerfiles.
I've also seen people use Nix or Bazel inside their Dockerfile, like ultimately the build has to execute somewhere, be that inside or outside a Dockerfile.
FYI, the nix2container [1] project (author here) aims to speedup the standard Nix container workflow (dockerTools.buildImage) by basically skipping the tarball step: it directly streams non already pushed layers.
I've also seen people use Nix or Bazel inside their Dockerfile, like ultimately the build has to execute somewhere, be that inside or outside a Dockerfile.