Correct command to execute container registry

[luizhc]

After stop container of the Registry to running the garbage collect, the command "run" not work in the docker for containers, beacuse is used only for images.

For use in container, follows command correct:

docker exec -t name-container-registry bin/registry garbage-collect /etc/docker/registry/config.yml

Note: the command above it worked correctly, however not is possible stop container and this can
generates images corrupts during the process according issue #65

Would be interesting update the file README.md of this project with that information?