Update docker compose examples to move way from deprecated `--link`

[tippexs]

First of all this repository contains awesome examples and getting started configuration! Well done!

I was just working on a issue https://github.com/nextcloud/docker/issues/1063 and updated my docker compose files to the latest API Version 3 and removed the deprecated --link option. As it is still supported in Version 2, Docker recommends the ussage of networks. See: https://docs.docker.com/compose/compose-file/compose-file-v2/#links

Before creating a PR I just want to be sure they are welcome and an update to compose V3 is appropriate! Happy to drop a PR to update the README.md docker compose exmaples.

https://github.com/nextcloud/docker/blob/master/README.md#base-version---apache

Keep up the great work! Cheers Timo

[tippexs]

@joshtrichards thanks for adding the labels and sorry for my late response! I will work on the PR and submit it once I am back in my office. Currently in transit over the atlantic ocean. ✈️