Setting Vaadin default locale via Java VM args?

Folks;
as I had a bunch of issues running a Vaadin application in docker, I finally managed to resolve this: The Vaadin app features a resource bundle ui_de.properties which obviously gets loaded per default on all our development and production machines (where apparently the locale is set to de_DE). Inside the docker container, the locale obviously defaults to some sort of "en", which makes Vaadin want to load ui_en.properties (which is not there).
Two questions:

• Can I set this locale in some way via Java command line arguments? stackoverflow suggested using something such as -Duser.language=de_DE, but this doesn't seem to work.
• More generally: Why does Vaadin honour the locale of the operating system it runs on in the first place? Shouldn't locale in a web application be set according to the Language headers provided by the requesting client? ;)

Thanks in advance and all the best,
K.