OpenFOAM is not a monolithic GUI program. It is a library and a set of
applications built on it. See http://openfoam.org, which is mentioned
in the package description. Ignore everything they say about installing
it, though. You have installed it. You certainly don't need Docker.
You should be able to run applications from the command line. Look in
/usr/share/doc/openfoam-examples. The documentation is abysmal. You
have to puzzle it out from clues on the Web site. There is a tutorial
somewhere: perhaps you can find it. The version of OpenFOAM in Sid
(which I use) is broken.