docs/devel/testing: Add -a option to usermod command on docker setup

docs/devel/testing: Add -a option to usermod command on docker setup

The option -G of usermod command will remove user from other groups
not listed, i.e.: $USER will belong only to group 'docker' after
following the documentation as is.

From usermod(8) manual page:

    If the user is currently a member of a group which is not listed,
    the user will be removed from the group. This behaviour can be
    changed via the -a option, which appends the user to the current
    supplementary group list.

This patch improves the situation by adding the -a option to the
usermod command, which will just append user to the supplementary
group list.

Cc: qemu-trivial@nongnu.org
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Message-Id: <20190207184346.6840-1-muriloo@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>