XPS 13 9380, D6000 dock, Ubuntu

The D6000 uses DisplayLink technology for video output (not to be confused with DisplayPort).  Make sure you've got the latest Ubuntu DisplayLink drivers available here if you haven't already.  Otherwise, the D6000's documentation says that if the system doesn't have DisplayLink software installed, the HDMI output should work through DisplayPort Alt Mode (i.e. a native GPU output), but maybe there are issues with that mode when that dock is used with Linux systems?

Another option you might want to seriously consider is dumping the D6000 and getting a dock that always taps into native GPU outputs rather than using DisplayLink, because DisplayLink can have some significant drawbacks that I wrote about in detail in this thread, specifically the post marked as the answer.  Even though the question was about gaming, the answer wasn't.  Other "native GPU output" docks you might want to consider if you have an XPS 13 9380 are the Dell WD19 and WD19TB.  The main difference is that with a 9380, the former dock can do dual displays up to 1920x1200 or a single 2560x1600 display, whereas the latter can do dual displays up to 4K 60 Hz and some triple display configurations.  Unless you specifically need the D6000 in order to run triple 4K displays and/or because you also want to use it with some other laptop that only has USB-A ports, I'd strongly recommend getting a different model.