docker-io 1.4.1-1.el6 which is on epel6 testing is adding sub-package docker-io-fish-completion which requires fish. Unfortunately fish is not available on epel6. I suggest disabling this sub-package on epel6.
I have removed the fish dependency but kept this package enabled here: https://admin.fedoraproject.org/updates/docker-io-1.4.1-2.el6 . I'll leave it upto the fish owner to provide fish on el6 or upto the user to install fish if they like.