You have plus model, so it should support virtual machines. Install VM manager and then install HAOS. Some guidelines are HERE. Although that topic is old VM settings are still valid. This way you’ll have fully working HA with all functions.
I’ve had HA installed in VM for a few years. I remember that i tried once and install HA in docker, just out of curiosity and if i remember correctly running speed wasn’t that much of a difference. Not to be noticed in any way. What’s better in VM is you have supervised HA with all functions, which is not the case in docker (at least not officially).
Oh…another thing: i hope you didn’t change default btrfs file system with ext4 when you bought / first started your syno - since if you use ext4 you can’t install VM manager. Then your only option would be to delete and re-create entire volume (that’s what i’ve had to do… 
)