Ok, this is just annoying now, and WD would do well to provide some additional guidance to the community here. Docker on the PR2100/PR4100 works just fine for me (YMMV).
It does, however, require the install of an “app” to enable it. The app can be found here: OS5 packages · Issue #57 · WDCommunity/wdpksrc · GitHub (specifically at this link: https://github.com/WDCommunity/wdpksrc/files/5152595/docker-19.03.8-OS5.zip)
It appears the specifics of this thread are primarily related to the EX4100 device which apparently no longer supports Docker.
