Document optimising multi-member workspace dependency management

[bengsparks]

When reviewing the list of dependencies that are added to a virtualenv when using

default = util.mkApplication {
  venv = pythonSet.mkVirtualEnv "application-env" workspace.deps.default;
  package = pythonSet.hello-world;
};

you will find that if, you have more than one member in your workspace, all dependencies from all members of the workspace are added. This is not an issue in local development, but will bloat the size of containers when attempting to used the build program for an image, e.g. nix2container or dockerTools.

Instead, by specifying the name of the package that you want your container to hold, you can shrink this down to only what is necessary.

default = util.mkApplication {
  venv = pythonSet.mkVirtualEnv "application-env" {
    app = [ ];
  };
  package = pythonSet.hello-world;
};

NOTE: The content of this PR is adapted from #168 (comment), but documenting something like this explicitly (and perhaps in more places?) would really improve the deployment experience.