Add descriptions of images in dockerhub

[dingp]

Alessandro mentioned that there are many tags for dunedaq/sl7-minimal and dunedaq/c8-minimal docker images on docker hub. The tag names are not self-descriptive. It would be helpful to add some description.

I have removed outdated image tags from dockerhub. Will need to add some documentation in this repo (and the copy it over to docker hub) to describe the contents of the images and their intended use.

All of the image tags, except latest are meant to be used by SW Coord for CI jobs. So this documentation is mainly for SW Coord team rather than general audience.