rebuild.sh for sdkman and jenv

[hboutemy]

for initial ease of use, rebuild.sh uses Docker images: it automatically imports the JDK version required by the buildspec
the downside is that de-facto, the rebuild is happening only with the Linux runtime provided by Docker images, whatever the OS you're running
switching between JDK versions is the key requirement for rebuild.sh: we can do it in a quite portable way with sdkman or jenv
Let's add an option to rebuild with sdkman or jenv, and let everyone build on their own OS if they want to: that will provide wider environment coverage for testing reproducibility