I was a bit worried that the F5-221 would come without an HDMI port, making it more difficult to install another OS. It turns out it does have an HDMI port (which was missing in the product pictures) but also that it was not needed in the end. It was as easy as cracking the NAS open, removing the USB dongle and putting a pre-installed USB key. As explained here or here, the key has to be very compact, I went for a 16 GB SanDisk Ultra Fit. I also added 8 GB of RAM, making it 10 GB of RAM in total.
It was a bit more difficult to understand how to get the Docker "app store" working because it is actually something you need to install yourself.
Basically, paste the following URL into the Plugins / Install Plugin section:
https://raw.githubusercontent.com/Squidly271/community.applications/master/plugins/community.applications.plg
I then immediately wanted to install a VPN and a BitTorrent client, and I also had a bit of difficulty there: I followed this YouTube video which is quite clear; but it never worked properly. I didn't notice the LAN netmask was wrong until I read this Unraid forum thread!