This PR adds the ability to add extra entries to /etc/hosts on the nextcloud-aio-collabora container.

I need this to work around an issue that I haven't yet fully resolved, to do with Collabora (in response to a request coming via coolwsd) attempting to retrieve config from my instance's external domain, rather than the internal domain. (This may be nextcloud/richdocuments#4906.) I intend to properly debug this and provide a PR later. But this functionality could be useful to others, e.g., #2395.

I didn't understand the function of update-yaml.sh, nor do I understand what the configName is for, so please let me know if I have missed anything.

This would probably be better if it was generalised to allow specifying for any container rather than only nextcloud-aio-collabora. But I didn't know the best way to represent this in config.

Thanks!