I have read here https://github.com/Microsoft/CNTK/wiki/Image-reader that the Image Reader supports ... as a notation for the directory where the map file is located.
I find it a little odd to use ... and not just a single dot as a notation for the current directory. I think I tried to use . in my first attempt and that it didn't work, but I'm not 100% sure.
What is the background for not just using the single dot as everyone is used to?
Single, double, and triple dot are all available. Single and double dot have the regular meaning (current directory or parent of current directory) of the process that reads the file which does not have to be the same as the directory in which the file currently is. As you correctly noticed, triple dot is relative to the map file and has nothing to do with the current directory of the process that opens/reads the file.