Add more detail topic about downloading docker images (elastic#7615)

libbeat/docs/shared-docker.asciidoc

@@ -2,7 +2,23 @@
 === Running {beatname_uc} on Docker
 
 Docker images for {beatname_uc} are available from the Elastic Docker
-registry. You can retrieve an image with a `docker pull` command.
+registry. The base image is https://hub.docker.com/_/centos/[centos:7].
+
+A list of all published Docker images and tags is available at
+https://www.docker.elastic.co[www.docker.elastic.co]. The source code is in
+{dockergithub}[GitHub].
+
+These images are free to use under the Elastic license. They contain open source 
+and free commercial features and access to paid commercial features. 
+{xpack-ref}/license-management.html[Start a 30-day trial] to try out all of the 
+paid commercial features. See the 
+https://www.elastic.co/subscriptions[Subscriptions] page for information about 
+Elastic license levels.
+
+==== Pulling the image
+
+Obtaining Beats for Docker is as simple as issuing a +docker pull+ command
+against the Elastic Docker registry.
 
 ifeval::["{release-state}"=="unreleased"]
 
@@ -18,11 +34,11 @@ ifeval::["{release-state}"!="unreleased"]
 docker pull {dockerimage}
 ------------------------------------------------
 
-endif::[]
+Alternatively, you can download other Docker images that contain only features
+available under the Apache 2.0 license. To download the images, go to 
+https://www.docker.elastic.co[www.docker.elastic.co]. 
 
-The base image is https://hub.docker.com/_/centos/[centos:7] and the source
-code can be found on
-{dockergithub}[GitHub].
+endif::[]
 
 [float]
 ==== Configure {beatname_uc} on Docker