Thanks David! Your quoted thread does help me out of the blue. I have replaced the docker run option "-u app" to "-u 0" (uid can be any number, but invalid number will have no associated user) or "-u root". Now the container can be started with no error:
docker run -i -t
--net=host
-e LC_ALL=en_US.UTF-8
-e LANG=en_US.UTF-8
-v /home/appadmin:/app -w /app -u root gaia
So, "app" user never exists inside the container, I just wonder whether this is it or the docker build process runs into trouble creating "app" user.