But a lot of those unofficial docker images are of unknown quality and could easily contain trojans. It's completely different from installing a package from your distro.
On the plus side, the Dockerfile and the repo with the scripts used to build a container is usually available. If you don't trust it, read through the source and rebuild it. Or just stick to official containers, no matter how terrible they are.
So don't use unofficial ones. :)
Plenty of primary sources package their software for Docker these days.