After a question in forum on how to collect docker container logs with elastic agent I stumbled across a potential issue.

As we do not have a specific integration to collect docker logs, we rely on custom-logs which uses logfile input type.

Leveraging the docker dynamic provider variables and setting log path to /var/lib/docker/containers/${docker.container.id}/*-json.log multiple inputs in agent per container are created as expected.
Although the log path on those inputs is correct , agent/filebeat does not collect the logs at all.

Steps to reproduce:

1. Deploy elastic stack on Elastic cloud with version 8.0.0-alpha
2. Set up a kubernetes environment with docker runtime. minikube start --kubernetes-version=v1.21.3 --container-runtime=docker
3. Configure inside elastic-agent-standalone-manifest.yaml agent.yml to collect the docker container logs

- name: docker-log
  type: logfile
  use_output: default
  meta:
    package:
      name: log
      version: 0.4.6
  data_stream:
    namespace: default
  streams:
    - data_stream:
        dataset: generic
      paths:
        - /var/lib/docker/containers/${docker.container.id}/*-json.log

4. Deploy standalone agent with image docker.elastic.co/beats/elastic-agent:8.0.0-SNAPSHOT. Filtering the logs in Kibana with event.dataset:generic , no logs are shown
5. Exec in agent container and run ./elastic-agent -c /etc/agent.yml inspect output -o default | grep docker, we can see one input per docker container. This means that the docker.container.id variable is populated by the docker dynamic provider.

name: docker-log
- /var/lib/docker/containers/06fd6e8770a51b700f2abaa584479ab22e4733953edf30be919a8f00da0c6814/*-json.log
        io_kubernetes_docker_type: container
        maintainer: NGINX Docker Maintainers <docker-maint@nginx.com>
name: docker-log
- /var/lib/docker/containers/0a07c2e613a301b2669e3f90dde2dd454000ea3829644748c8a0a279114dc9b4/*-json.log
        io_kubernetes_docker_type: container
name: docker-log
- /var/lib/docker/containers/0e30f4a7cc42ef16219a108911079fbe1734ffc2c7a19c06c995d173808dc936/*-json.log
        io_kubernetes_docker_type: podsandbox
name: docker-log
- /var/lib/docker/containers/1f58c8b2eefe5889716786910544223a3072493e2eac7676b2aa2460eb199d63/*-json.log
        io_kubernetes_docker_type: container

6. Although those log paths are correct and full of logs, filebeat does not collect the logs or log anything supsicious.
7. If we update the agent.yml configmap removing the dynamic variable and restart the agent then logs are collected

- name: docker-log
  type: logfile
  use_output: default
  meta:
    package:
      name: log
      version: 0.4.6
  data_stream:
    namespace: default
  streams:
    - data_stream:
        dataset: generic
      paths:
        - /var/lib/docker/containers/*/*-json.log

This is very weird and it seems that either the logfile input is not working as expected(although I tested the same with filestream input and did not fix the problem) or the agent-filebeat communication may have a problem.
Or something different.

Also the documentation of docker dynamic provider is outdated. Variables like docker.id do not exist. Instead the provider populates docker.container.id.
We can see the mapping it creates per event here.