TL;DR: Install docker AND dockerd
Unfortunately the Wiki documentation appears out of date for installing Docker as a container host in OpenWRT.
The majority of the documentation is fine. However, the instructions tell a user to download 'docker-ce' with opkg, which is no longer possible and presumably out-of-date. The correct package(s) (assuming you also want docker-compose), are:
`docker dockerd docker-compose`
The older docker-ce package seems to have been broken out into the CLI tools ('docker') and the daemon/engine ('dockerd').
Hope this helps someone and I have requested access to the OpenWRT wiki to make these changes.