support/docker: don't use sudo to build

On a properly setup machine, it is totally useless to use sudo to run
docker; it is very bad practice. Instead, users really should add
themselves to the docker group.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>