Introduce ECS_PAUSE_LABELS env var to apply labels to pause container

[shogondo]

Summary

This change introduces new environment variable ECS_PAUSE_LABELS. This can be used to apply custom labels to pause containers. Related issue: #4427.

Implementation details

Changes:

• Update task.DockerConfig to apply custom labels to pause containers if the ECS_PAUSE_LABELS provided.
  • ECS_PAUSE_LABELS can be specified with JSON format, similar to ECS_AGENT_LABELS
  • If ECS_PAUSE_LABELS isn't provided, does nothing.
  • If ECS_PAUSE_LABELS is invalid format, just ignores it.

Testing

Tested manually on an EC2 container instance.

Config:

$ cat /etc/ecs/ecs.config
ECS_CLUSTER=my-sample
ECS_PAUSE_LABELS={"test.label":"hello world"}
ECS_AGENT_LABELS={"mylabel.1":"value1","mylabel.2":"value2"}

Before:

$ docker inspect CONTAINER_ID | jq .[0].Config.Labels
{
  "com.amazonaws.ecs.cluster": "my-sample",
  "com.amazonaws.ecs.container-name": "~internal~ecs~pause",
  ...
}

After:

$ docker inspect CONTAINER_ID | jq .[0].Config.Labels
{
  "com.amazonaws.ecs.cluster": "my-sample",
  "com.amazonaws.ecs.container-name": "~internal~ecs~pause",
  ...
  "test.label": "hello world"
}

New tests cover the changes: yes

Description for the changelog

Enhancement: Add new environment variable ECS_PAUSE_LABELS to apply custom labels to pause containers

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

Does this PR include the addition of new environment variables in the README?

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[danehlim]

Thank you @shogondo for your contribution with this pull request! I have made some updates to this pull request to address the comments I raised and will get this pull request reviewed by another member of our team.