In Fedora CoreOS (in a virtualbox VM), with latest images of docker.io/x11docker/xserver and docker.io/x11docker/fluxbox, using the 7.4.2 version of x11docker script:
x11docker -D -V --backend=podman --desktop x11docker/fluxbox
loops apparently forever with:

...
DEBUGNOTE[16:27:10,027]: waitforlogentry(): tailstdout: Waiting since 703s for log entry "x11docker=ready" in store.info
DEBUGNOTE[16:27:10,027]: waitforlogentry(): tailstderr: Waiting since 703s for log entry "x11docker=ready" in store.info
...

I'm assuming docker.io/x11docker/{xserver,fluxbox} are your images. Am I wrong? Also, doesn't the x11docker script test such things?