Docker file was added and the readme file was updated

For now, it should be "optional" but we should consider making it default as each image in a container is fully isolated from other software, so we should use the latest versions of Java as a default.

While I agree with the sentiment, given current rapid-ish development cycle for java IMHO it would be better to stick to version 17, which is "LTS" (in quotes, as there is not such thing as LTS but certain providers may provide it, and AFAIR adoptiom is going to push their changes upstream, which effectively would make it LTS).

For now we can use "16" and switch to 17 once it's released soon.

@wojtek The idea was to have 2 builds/images at all times:

• newest stable (JDK16 now)
• latest LTS (JDK 11 now)

The idea was to have 2 builds/images at all times

AFAIR we aimed for having Tigase relatively most compatible (so we stuck with JDK8 for longer) and added JDK11 at one point because it was required by tigase-push APNS dependency, but given it was commercial component and JDK11 was already LTS we went ahead with it.

On the other hand, you are now proposing having "newer java" to be one of the fast paced released. Which isn't all bad, especially in Docker context where it's independent from underlying OS, but it imposes more maintenance work on us - those version have only 6 months of support so we would have to update those docker images at least. On the other hand - we probably should have been building "LTS" images to update to the latest bugfixes (of version 11 currently).