Add dummy "user.email" setting in our Dockerfile for smooth in-container merge commits

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Use lxc-stop -k when lxc-kill is not found

lxc-kill was removed in lxc/lxc@33ddfc2

Docker-DCO-1.1-Signed-off-by: Chia-liang Kao <clkao@clkao.org> (github: clkao)
Merge pull request #3844 from clkao/lxc-kill-deprecation

Use lxc-stop -k instead of lxc-kill
Merge pull request #3828 from tianon/add-user-email-to-dockerfile

Add dummy "user.email" setting in our Dockerfile for smooth in-container merge commits