Grading environment based on grading-base with specific version of MiniZinc.
Images are tagged with MiniZinc and grading-base versions in format <minizinc>-<grading-base>.
Version tag can also include uN meaning update N where N is an increasing number.
The update part is used to indicate updates to the image, where software versions did not change.
For an example, 2.1.7-2.0u1 includes MiniZinc 2.1.7 on top of grading-base 2.0 and has one update after first release.