Custom CI with Docker page should contain the list Convinience image tags

[shiroyasha]

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[markoa]

The problem here is that our docs say you should write image: semaphoreci/lang:<version>, and to see which versions are available, well go read the Tags page which lists unsorted version numbers on 10+ pages. :)

We should follow the standard of listing currently supported version names on top of the Readme, such as on https://hub.docker.com/_/redis.

[CommanderK5]

@markoa @shiroyasha We could do this but we have to enable docker hub integration with semaphoreci org in github. Dockerhub will automatically update repository's readme. Other option is to publish a list of all supported tags somewhere in the docker-images repos.