Fix docker mounts to allow the riak docker container
to be started nested.
Also note about nested containers in the docs.
Related-bug: #1548851
Related-bug: #1547587
Change-Id: I2fb448ac439b717eae1c3deee2cd067ac8b4f0c7
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>