motivation: setup for testing across different permutations of linux and java versions
changes:
* refactor docker file to allow dynamic versions of centos and java
* add docker compose driver files for centos 6, 7 and java 1.8, 1.9, 1.10, 1.11
* update instructions
Motivation:
It would be good to provide a docker image for people that want to build netty on linux.
Modifications:
Add a docker file
Result:
People can more easily build netty. Fixes [#7585].