enable PLANTUML_LIMIT_SIZE as Docker environment variable

[bufferoverflow]

Would be nice if PLANTUML_LIMIT_SIZE could be used as Docker environment variable in the same way as on command line.

[Ducatel]

Hi,
I did some test and it working without any update

docker run -ti --rm -p 8080:8080 -e PLANTUML_LIMIT_SIZE=4567 plantuml/plantuml-server:jetty

As you can see, the value of PLANTUML_LIMIT_SIZE was correctly setted.

[bufferoverflow]

Thank you!