I have a water simulaton with the container in my dopnet node.
How can i bake out the container geometry from the dopnet node?
Do i use an dopimport in the sop?

dop import brings it into the geometry level, yes. This will be mandatory for any part of dops you wish to render (although it can be done with an object merge node as well).

then use a rop_geometry node to cache it to disk, and a file node to bring it back in.