It depends of the other things you use your Syno for, but normally the DS218+ is more than sufficient for the average user. When running Docker, enough RAM is very important like @robertklep says.
When you want to run multiple containers or containers that need a lot of allocated RAM, expanding the RAM is a must. Therefor a DS218+ with extra RAM is a good buy, especially if you want to spend your money on more disk space. A DS718+ is better, but not needed for some basic stuff and running Docker only.
(I own a DS718+ myself)
Edit:
When only running a HA container, extra RAM isn’t needed.
