docker.userguide: Add --network=host option for manual docker build examples

docker.userguide: Add --network=host option for manual docker build examples

On Ubuntu hosts (16.04 & 18.04) there might be problem to build
docker collectd container if host is behind a proxy even if
proxy variables are set correctly - when using "--network=host"
option problem is resolved.
This option has no negative impact on other host systems so adding
it to default command line build example.

Change-Id: I69ebd13f1e05658d8ad18e3ec379f515312e7b74
Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>