aws-ecs-1: enable SELinux capability

[samuelkarp]

Issue number:
#815

Description of changes:
Configure the ECS agent to register the SELinux capability so SELinux options can be set in the task definition:

"dockerSecurityOptions": [
    "label:type:super_t"
]

Testing done:

• Ran a task through ECS with a restricted host path (/etc) mounted and no explicit SELinux options. Observed that the container received the default container_t label and that it cannot write to the /etc bind-mount.
• Ran a task through ECS with a restricted host path (/etc) mounted and the super_t label assigned in the task definition. Observed that the container received the super_t label and that it can write to the /etc bind-mount.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jahkeup]

Looks good! 🔐 🐳