lxc container : password and username

On newer installs, at least on my current install, you get this message instead of the old ubuntu/ubuntu message: (emph. mine)

You just created an Ubuntu container (release=trusty, arch=amd64, variant=default)

To enable sshd, run: apt-get install openssh-server

For security reason, container images ship without user accounts and without a root password.

Use lxc-attach or chroot directly into the rootfs to set a root password or create user accounts.

so how do you create a new user on a new LXC?

As hinted, you can either chroot into the rootfs, of use lxc-attach, which is the easiest option:

you@hostbox:/$ lxc-attach -n yourlxc
root@yourlxc:/# adduser username