* Add simplified Docker Compose tutorial
references [issue #1903](https://github.com/iv-org/invidious/issues/1903)
Revised Docker walkthrough in `Installation.md`
Includes up to date Docker Compose file with Quay image replacing the now-defunct Docker Hub image
Note: In the Docker Compose file, the POSTGRES_USER env var can't be changed because the username `kemal` is hard coded into the initial DB migration files found in `./config/sql/`
* Replaced link to gist with contents of Compose file
