You might be better off to ask this question in a Synology forum, as Docker itself does not provide a release for any NAS. See Install Docker Engine | Docker Documentation for supported platforms.
This one is easy to answer. According https://kb.synology.com/en-me/DSM/tutorial/What_kind_of_CPU_does_my_NAS_have your NAS has a Realtek RTD1619B CPU ( ARMv8 architecture), and Synology never released a Docker package for any other CPU architecture than x86_64. All “+” series have a x68_64 architecture CPU. The “non plus” DS223 does not.
You are better off to buy a Raspberry Pi and run docker on it, than trying to hack together a solution to make docker work on your NAS.