It's not really without a UI, it's starting minimized and with no blocking error boxes during that run.
Good to know. No blocking error boxes is useful. Should keep better track of these things. That´s one of the options that I mainly kept around as default because "it´s always been this way". I frankly forgot why I ever put it there. I just kept it consistent.
You only need to use the BuiltInBrowser switch once and only if you can't start MC after installing. That problem will probably go away in the next build of MC.
Will wait for the next version to build docker images then. If people are unpatient the image isn´t that hard to build. Just run
git clone https://gitlab.com/shiomax/jrivermc-docker.git
git checkout dev
./build.sh --jriver-tag latest mc34
Then use "shiomax/jrivermc-testing:mc34-v0.4.6-amd64" as the image (or read the console output and use that, should be the same if you do it at the time of writing this) and set MC_STARTUP_OPTIONS to /BuiltInBrowser once.