Add info about official Beats docker images

[dedemorton]

Adds info about official docker images to the metricbeat docs. This note will only show up if the release state is set to "released" in versions.asciidoc.

[andrewkroh]

I think it would be good to have a link to the beats-docker repo. People will be interested to see the contents of the Dockerfile.

And yes, I think we need a similar topic for all Beats that have Docker images. Each of Beats have some specific things we could document.

• Packetbeat - how to monitor the host's network traffic, what capabilities to add to the container so that it can run as non-privileged (cap_net_raw cap_net_admin)
• Filebeat - how to monitor the host's logs, how to monitor docker logs

[dedemorton]

@andrewkroh I've added a link that points to the beats-docker repo.

I've also created #4192 to get the other beats doc updated with info about running in a container.

[andrewkroh]

jenkins, test it