It looks like Docker support is still a paid feature? That's disappointing. I had written a blog post outlining how important docker support was and explaining how little work it was to get working, and at the time the maintainers agreed it should be a free feature, but it seems they've changed their minds.
If it's using nix for building - there's multiple ways to DIY turn that into a docker container, like pkgs.dockerTools.buildImage or nixpacks: