Create single tasks per target platform

[Vampire]

It would be nice to have a single task per target platform, so that you can also build the images individually.
Currently if I want to test how the program behaves under the stripped down JRE (already had very strange behavior due to missing modules) you by default have to build for all target platforms, including download of the foreign JDKs just to run the current one.

So having separate tasks per target platform would be much nicer (and also allow for parallel execution of those once Gradle finally finished implementing that). Then you can targeted build only a subset of the full potential and you could still provide tasks that are named generically like the current ones that are lifecycle tasks, meaning they depend on the individual ones to run them alltogether.

That would also make the "release version check" you see in my example in #99 superfluous as it is mainly there to not require to download JDK and build for foreign platform when I just want to test on the current one.