An example of a Docker image created using a `default.nix` (which could, in the future, be generated by Dockter). This illustrates how someone can create a Docker image using Nix, without having Nix installed on their host system.

It uses `pkgs.mkShell` to make Python available to execute `main.py`. A better approach would probably be to use `stdenv.mkDerivation` to create a derivation for the project sources and then include that in the `buildInputs` to `pkgs.mkShell`: that way the project becomes a package, but there is still a shell to drop into for debugging.

An alternative way of creating a Docker image is to use Nix's `dockerTools.buildImage` (like we do in the [`environs`](https://github.com/stencila/environs/blob/master/nix/docker.nix) repo), but that requires Nix to be installed on the host system.

It would be interesting to see how the image sizes differ between the two approaches.