Add live-reload for go files

[TrevorEdris]

This is a minimization of the changes from https://github.com/mikestefanello/pagoda/pull/2. I've used this template for quite a few other projects and these changes are always the first update I make to the project, so I figured others may find them useful as well.

The main change here is the live-reloading of any changed .go files, triggering the auto rebuild and rerun of the web app running in docker compose. This is achieved via https://github.com/cosmtrek/air.

[mikestefanello]

This is now a bit outdated so I'm going to close. I don't use air, but If you're interested in re-opening, I would be fine including support for it.