Docs: Promtail Docker config example seem out of date with the docs

[aocenas]

Describe the bug
In docs example section there are parts:

pipeline_stages:
   - docker:

but docker does not seem to be a valid option for the pipeline_stages. I would expect kubernetes_sd_config there.

To Reproduce

Expected behavior

Environment:

Screenshots, Promtail config, or terminal output

[slim-bean]

hey @aocenas I agree that example is confusing and misleading. The config is technically valid if you had docker writing logs into /var/log but this is very OS dependent and we really recommend the docker logging driver instead.

docker is a valid pipeline_stage but was not documented which is also not very helpful.

I opened #1602 to hopefully address the problems you encountered.