Nextcloud(docker) on Ubuntu with existing (native isntall) nginx setup

🚧 Installation
1

I host multiple websites on my server running Ubuntu 18.04.

I have nginx natively installed and running smoothly.

My question is two folds:

PART 1:
Is it possible to host a nextcloud instance on that same server and preferably using Docker?

The issue is that the NATIVE nginx service is already listening on the TCP 443, and serving websites store in /var/www/

PART 2:

The next thing that got me confused is:
“volumes:
- nextcloud:/var/www/html”
"
What I understood is that nextcloud would store the files in this directory(for persistancy), correct? If so, what happens to the already stored websites in both /var/www and /var/www/html?

Any help is appreciated. Cheers