Fix docker provider add_fields processors

[belimawr]

This is a backport PR for: elastic/elastic-agent#1420

What does this PR do?

The Docker provider was using a wrong key when defining the add_fields processor, this causes Filebeat not to start the input and stay on a unhealthy state. This processor seems to be defined every time a variable like ${docker} is used on a input configuration.

This commit fixes it.

Why is it important?

It fixes a bug on Docker auto discovery provider

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Decide about backports (7.17?)
• Backport to 7.17.x (PR on Beats repo)

How to test this PR locally

1. Make sure you have Docker installed and running
2. Deploy Elastic Stack
3. Start a container that will generate logs (e.g: docker run --rm --name flog -d mingrammer/flog /bin/flog -s1 -d1 -l -f rfc3164).
4. Create a standalone Elastic-Agent policy using Kibana as suggested in the documentation.
5. On this policy add the "Custom Logs Integration" configuring the path as /var/lib/docker/containers/${docker.container.id}/*-json.log.
6. Download the policy (e.g: /tmp/elastic-agent.yml), adjust the Elasticsearch credentials as needed.
7. Start the Elastic-Agent with log debug (running as root is usually required to read /var/lib/docker/*):

   sudo elastic-agent  -e -d "*"
   

8. You should see logs on Kibana

Related issues

• Fixes Elastic agent is not collecting docker container logs with the use of docker dynamic variables #29030

## Use cases
## Screenshots
## Logs

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

[elasticmachine]

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Build stats

• Start Time: 2022-10-11T09:19:10.776+0000

• Duration: 110 min 24 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6209
Skipped	394
Total	6603

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[belimawr]

/package

[belimawr]

/test

[belimawr]

/test

[belimawr]

/test

[belimawr]

I'll merge this because only the e2e tests are failing and they're know to be flaky.