`docker` input only works on filebeat < 8

[widhalmt]

Release 8.x of Filebeat doesn't have a docker input anymore. We need to provide a working solution for 7 and 8.

Please make sure they are correctly documented (without the current warning) in docs/role-beats.md. And the checks reenabled in the beats_peculiar Molecule scenario.

[widhalmt]

NOTE: The warning is currently missing and the check is still activated. But this will change as soon as #55 is merged.

[lcndsmr]

Is this still up to date?

[widhalmt]

Honestly, I lost track. :-( The mentioned PR is merged. So I guess, it's fixed. I'll close the issue. Please keep docker input in mind. If we hit it again, we can re-open the issue.