Services started by systemd in a gui-enabled container didn't have
WAYLAND_DISPLAY set, so Wayland clients couldn't find the compositor.
libwayland would fall back to a headless display or error out, leaving
apps running invisibly while the VNC session showed a blank weston
desktop (the double-screen problem).
Fix in weston-vnc.nix:
- Pass --socket=wayland-0 to weston so the socket name is
deterministic (weston normally picks any free wayland-N name).
- Set WAYLAND_DISPLAY=wayland-0 and XDG_RUNTIME_DIR=/run/user/0 as
global environment.variables gated on hyperhive.gui.enable, so
every service in the container inherits them automatically.
- Update gui.json to include wayland_display for tooling that reads it.
Update docs/gotchas.md with the rationale and pointer to #540.
8a97277f20