You will receive that error message either when when attempting to connect to a TLS (secure) socket without using TLS or attempting to connect to a non-TLS secured socket with TLS.

The configuration in /etc/default/docker may not be sourced in certain situations. It is an implementation detail of the init system (e.g. newer version of Ubuntu with systemd will not use this file). The best approach is to use Dockers own configuration files.

In /etc/docker/daemon.json, set:

{
  "insecure-registries" : ["example.com:5000"]
}

For more info (and the requisite warnings about using an insecure registry), see: https://docs.docker.com/registry/insecure/