Document how to run Beats in a container

[dedemorton]

We currently document how to run Metricbeat in a container, but we don't provide this info for other Beats. We need to document this info for all Beats and provide specifics. For example:

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

There will be some shared content, so we probably want to use a shared file in libbeat. The content that's unique to a specific Beat can be conditionally code.

[exekias]

@Jarpy opened this PR #4312

[dedemorton]

Fixed in #4312