- [x] Have you followed the guidelines for contributing? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [x] Have you successfully run `make lint && make test`? --- This PR modifies the "adding an internal user" how-to guide to "creating a rootless rock" and present two approaches: * using `run-user` field in rockcraft.yaml (previously undocumented) * manually adding a specific user (content from the previous "adding an internal user" guide) --- See the [built docs](https://canonical-ubuntu-documentation-library--846.com.readthedocs.build/rockcraft/en/846/how-to/rocks/create-rootless-rock/)