All you should need to do to get a running grav install in a container is:

docker run -p 8080:8080 robertbak/grav

The apache in the container is serving grav on port 8080 to make sure you don't need any admin permissions to use the port.

The easiest way to install plugins is just setting the docker environment variable called INSTALL. This will run the gpm install when the docker container is run.

Example:

docker run -p 8080:8080 --env INSTALL="admin" robertbak/grav

Will result in a grav install with Admin Panel and dependencies installed availiable on port 8080.