I'm not sure if this is a ROS, Gazebo, or ros_gazebo issue, but I thought I'd see if anyone here has a solution or a good method to trouble shoot.

I've installed Gazebo 8 with ROS, and ros_gazebo in a docker container

Any roslaunch of the gazeboros package will open Gazebo, and seemingly start up ROS packages, but it won't actually launch the world file. To replicate, install Gazebo as described above and roslaunch gazeboros mud_world.launch. You'll see none of the models in this launch/world file show up.