G'day Joey.
In Docker.hs, line 73, docker.io is listed as the package to be installed.
Docker.installed currently fails for me on Stretch with:
E: Package 'docker.io' has no installation candidate
Unless I'm mistaken, from Stretch this is now replaced by "docker-engine".
I can't see any docker-engine package in any version of Debian. Unstable still has a docker.io, though testing does not (update: it does now; the docker package also recently got updated to a more current version). Docker was not included in the last stable release.
I have not used docker in quite some time. I use systemd-nspawn containers which are much easier to build and maintain. So, it may make sense to either mark the docker module in propellor as unmaintained, or find someone else to maintain it.
Marked the module as unmaintained. If you would like to take over maintanance of it, just send me patches putting your name in the maintainer slot etc.