docs: Advice to use the `sudo docker` in example

Basic security guidelines advice to use `sudo` to run docker. We have
the support for this via `--docker-cmd` so let's use it in the
demonstration example to make it more visible. Additionally this should
make it easier for people to just follow the example without the need to
configure their docker to run as non-root without sudo.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/RunningTestsRemotely.rst

@@ -168,7 +168,7 @@ the test execution to be properly performed.  There's one ready to use
 image (``ldoktor/fedora-avocado``) in the default image repository
 (``docker.io``)::
 
-    $ docker pull ldoktor/fedora-avocado
+    $ sudo docker pull ldoktor/fedora-avocado
     Using default tag: latest
     Trying to pull repository docker.io/ldoktor/fedora-avocado ...
     latest: Pulling from docker.io/ldoktor/fedora-avocado
@@ -182,7 +182,7 @@ Assuming your system is properly setup to run Docker, including having
 an image with Avocado, you can run a test inside the container with a
 command similar to::
 
-    $ avocado run passtest.py warntest.py failtest.py --docker ldoktor/fedora-avocado
+    $ avocado run passtest.py warntest.py failtest.py --docker ldoktor/fedora-avocado --docker-cmd "sudo docker"
     DOCKER     : Container id '4bcbcd69801211501a0dde5926c0282a9630adbe29ecb17a21ef04f024366943'
     JOB ID     : db309f5daba562235834f97cad5f4458e3fe6e32
     JOB LOG    : $HOME/avocado/job-results/job-2016-07-25T08.01-db309f5/job.log