support/docker: allow the br-user to becone root
When our Dockerfiles (and images) get reused by users, they might need
to locally add new packages or tweak the iamge to heir local needs.
As such, register the br-user to become root by running (exactly!):
$ sudo su -
This will allow acutally becoing root without prompting for a password,
but no other command will be accepted, and no other paramter will be
accepted either.
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc; Thomas Petazzoni <thomas.petazzoni@bootlin.com>