Provide convenience scripts for building different JDK versions

[Marcono1234]

As mentioned in #3, it would be useful to provide scripts to build the different JDK versions.
Currently this requires finding the appropriate Git JDK repository (see also #1 (comment)) and choosing an appropriate boot JDK.

Probably makes sense to cover:

• jdk8u
• jdk11u
• future LTS releases, see https://www.oracle.com/java/technologies/java-se-support-roadmap.html
• latest JDK

At the moment it looks like the same Dockerfile can be used to build both JDK 8 and JDK 16. However, in the future separate Dockerfiles might be needed.

An issue which could arise is that the necessary boot JDK's will not be available as Ubuntu packages anymore. In that case a workaround could be to allow specifying a boot JDK download URL, e.g. AdoptOpenJDK / Adoptium or Azul Zulu releases; Azul Zulu also provides DEB packages for download.