When you type docker run -it -v... IMG at your shell, docker runs the image IMG: this means that docker starts a linux virtual machine, whose disk image, config, etc is contained in IMG. You do this at the shell of your host machine.
The fenics image you are running doesn't know a word about docker, nor has an executable in it with that name.
Once you have run your image, you get a shell to it. This shell will have a prompt like fenics@a823742b. In docker terms, you have run the image to get a container holding the shell you are seeing.
This container / running image has a name, which you can see with the command docker ps (this again from the host computer). Container names are funny compositions of names of scientists and adjectives, like "outrageous_einstein" or whatever...
If you happen to exit the fenics shell or want another one, you use this name to run stuff in the same container: Again from your host computer, i.e. at its shell, type
docker exec -it -u fenics <container-name> /bin/bash
This can be used to start an ipython notebook, run scripts or whatever, all from the host computer.
