How to install and use Nextcloud on Synology NAS securely (Virtual Machine, Docker)

For years I have been using ownCloud on a Synology DS209 (currently: DSM 6.2) to synchronize my calendars and address books between Linux PC and laptop and Android phone (CalDAV and CardDAV). For this I have set up Dynamic DNS and port forwarding in the router. Now I have a Synology DS420+ with DSM 7 and want to do the same with Nextcloud, but how?

Would it be better to install Nextcloud in a virtual machine (guest: Ubuntu Linux)? Honestly, until now I only use VirtualBox locally to use old Windows 98 software with the PC.

Or should I use Docker? I have no idea about that at all yet.

Hints on how I can improve security without going offline are always welcome!