Provide example for how to use dotfiles with the cli

[leighmcculloch]

It's unclear from the CLIs help, and the documentation and examples in the repo how to use the cli to have dotfiles installed in devcontainers. It'd be helpful if there was an example for how to do that documented since it's a common feature use.

[ai]

devcontainer up \
    --dotfiles-repository https://github.com/ai/environment.git \
    --dotfiles-install-command devcontainer/install-dotfiles \
    ./