webtatic/centos6:base - A minimal image that only boots core services. No root password set and no users.
webtatic/centos6:server - A image that contains standard server services (sshd, postfix, crond etc). This can be used as a base image for system containers that act like real servers. No root password set and no users.
webtatic/centos6:vagrant - A image suitable for vagrant, based on the server image, which sets up a vagrant user with vagrant's insecure_private_key SSH key for direct use with vagrant docker provider with has_ssh enabled.
Source / Issue tracker - https://github.com/webtatic-dockerfiles/centos6