improve docker build script and add explicit build script to dockermaven project

• allow docker build file to override nifi.version value in dockerhub folder. Default value in master branch may not be right value to use (1.9.2 vs 1.10.0).
• have a build script in dockermaven and dockermaven-stateless projects for newbies. Explicit build script like dockerhub may help avoiding try-and-error.

I will shortly create a patch and submit a PR.