Missing option for repositoryCredentials

[waeltken]

"containerDefinitions": [
    {
        "image": "private-repo/private-image",
        "repositoryCredentials": {
            "credentialsParameter": "arn:aws:secretsmanager:region:aws_account_id:secret:secret_name"
        }
    }
]

https://docs.aws.amazon.com/AmazonECS/latest/developerguide/private-auth.html

[jc-asdf]

I've addressed this in #24; would love a review. I'm using my fork of it in a project and it works well.

[aknysh]

addressed in #24