[ECS] [request] Support for alternative docker runtimes

### Summary
Support for alternative docker runtimes by specifying `ECS_DOCKER_RUNTIME`.

### Description
To be able to use custom docker runtimes like [sysbox](https://github.com/nestybox/sysbox) is necessary to do some pretty involved configurations as showed in this post: https://aws.amazon.com/pt/blogs/aws-brasil/utilizando-runtimes-customizados-no-amazon-ecs/.

By providing an env variable like `ECS_DOCKER_RUNTIME` we could make the configuration easier by building an AMI based on Amazon Linux (1, 2 or 2023), installing sysbox and then setting `echo "ECS_DOCKER_RUNTIME=sysbox-runc" >> /etc/ecs/ecs.conf`.

Without that variable we're forced to start and manage the ECS agent manually as well as setting up network configuration with iptables.

Review the requested ECS_DOCKER_RUNTIME setting and the /etc/ecs/ecs.conf configuration shown in the issue, then consult the linked AWS post for the custom-runtime setup. Define how the ECS agent should consume the setting and verify that an ECS host using sysbox-runc can start workloads without manual agent or iptables management.

Written by the indexing model from the issue text.