Add section on cgroupns usage for metricbeat under docker

[fearful-symmetry]

Proposed commit message

This adds a section to the "running on docker" doc to mention the use of --cgroupns=host.

While monitoring for private cgroup namespaces technically works (after no small amount of effort), the basic use case of "monitor the host cgroups from within a container with a private cgroup namespace" requires a considerable amount of hacks and should probably be considered best-effort. This adds a section telling the user to select a host namespace.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[pierrehilbert]

Adding @blakerouse & @michalpristas as reviewers for the docker knowledges.

[blakerouse]

Looks good.

Completely makes sense that you would want the container to use the host namespace so the cgroup information is correct.