fix tag when pulling an image via docker run

Fixes #1119 

When docker run an unknown image, it fails if we specify the tag

    $> COUCH1=$(docker run -d -v /var/lib/couchdb shykes/couchdb:2013-05-03) 20:10:57
    Pulling repository shykes/couchdb:2013-05-03 from https://index.docker.io/v1
    2013/07/03 20:11:06 Error: No such image: shykes/couchdb:2013-05-03
    $>

Thanks @rogaha for noticing