Support for alternative docker runtimes

[mliezun]

Summary

Solves aws/containers-roadmap#2345

Implementation details

• Read variable ECS_DOCKER_RUNTIME.
• Store in types.Config new attribute: Runtime str
• When starting a task pass the HostConfig with hostconfig.Runtime = cfg.Runtime as read from env variable.

Testing

• Added unittests to check that the new variable is correctly read and passed to the docker client.

New tests cover the changes: yes

Description for the changelog

Feature - Support for alternative docker runtimes

Licensing

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

[sparrc]

Hi Miguel, could you explain how you are using this and how it's been tested? Are you currently using some alternative docker runtime successfully with ECS?

[mliezun]

Hi @sparrc, as explained in the linked issue, I was trying to use sysbox with ECS containers and found this guide: https://aws.amazon.com/pt/blogs/aws-brasil/utilizando-runtimes-customizados-no-amazon-ecs

It needed some tweaks to make it work with the latest version of Amazon Linux 2 but I got it working.

By modifying the agent itself to support alternative runtimes we make the process for configuring the AMI a lot easier. Instead of having to run the agent by itself, configuring docker and configuring the network we just need to set ECS_DOCKER_RUNTIME=sysbox-runc.