I think this is a reference to the old, deprecated DDSM (DSM via Docker), that is no longer supported in the latest versions of DSM/Docker.
As I said, this is not something that will work with the current version of the Docker package or DSM for that matter. If you want DDSM then you will need a specific version of Docker for Syno in order to have DDSM support in the 1st place.
If you do you will then be able to install it as normal, but not sure how you would migrate it as such considering that the installation process of DDMS is executed via UI.
In any case, using DDSM, in the long run, is something that you should consider moving away from considering that you will at one point hit a wall with Docker package and DSM compatibility. A newer version of the Docker package will ask for a specific version of DSM so if you want to be up to date, eventually you will have to upgrade and essentially lose DDSM support.
Way forward for a "virtual" DSM is using a similar process but via VMM package and booting up VirtualDSM. It's heavier on the resources than DDSM but it is the only official way atm that Synology supports.
More info on DDSM being deprecated:
Synology Community