docker-compose.demo.yml: DEMO env var for UI
docker-compose.demo.yml: DEMO env var for UI

Issues: MEN-893

Signed-off-by: mchalczynski <3a897b56ed6fc284ae11e3e7ec3ea48d098a7178@rndity.com>

GregorioDiStefano/integration,pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration