It looks like a container with the name qgis-desktop-2-4 already exists in the system. You can check the output of the below command to confirm if it indeed exists:
$ docker ps -a
The last column in the above command’s output is for names.
If the container exists, remove it using:
$ docker rm qgis-desktop-2-4
Or forcefully using,
$ docker rm -f qgis-desktop-2-4
And then try creating a new container.