[doc] Clarify Docker image requirement.

doc/index.rst

@@ -198,7 +198,12 @@ As you see in the `optional variables section <./index.rst#optional-environment-
 Pulling Docker image from an online hub
 +++++++++++++++++++++++++++++++++++++++
 
-You can use `DOCKER_IMAGE` variable for this.
+You can pull any `Docker` image by specifying in `DOCKER_IMAGE` variable, as long as the following requirement is met:
+
+* `python-catkin-tools`, `python-pip`, `python-rosdep`, `python-wstool`
+* sources.list set up (`example <http://wiki.ros.org/kinetic/Installation/Ubuntu#Installation.2BAC8-Ubuntu.2BAC8-Sources.Setup_your_sources.list>`_).
+
+If your Docker image is missing any of the above libraries, then you can still pass their name by `ADDITIONAL_DEBS` (see `variables section <./index.rst#optional-environment-variables>`_).
 
 Note-1. This disables the handling of `ROS_REPOSITORY_PATH` and `ROS_DISTRO` as ROS needs already to be installed in the image.