Edit Add-on Dockerfile for testing

Is it possible to edit the dockerfile for an add-on obtained via GitHub by any chance? If so where would I find this file after SSH’ing into Hassio please?

I would like to test installing some packages to see if they work before reporting back to the add-on developer.

Thanks.

Use the files from GitHub to create a local add-on. https://home-assistant.io/developers/hassio/addon_development/ shows you how to deal with local add-on.