(ecs): default to stable fluentbit image instead of latest

## :question: General Issue

### The Question
I am affected by a bug in Fluentbit that causes DNS problems when using firelens log driver, almost with datadog. (https://github.com/aws/aws-for-fluent-bit/issues/233). As of now, working solution is to not use latest image and use 2.19.0, which I have tested working.

My problem is that using CDK I don't see how I can set to use a fixed fluentbit version in the "log-router" container, as that container definition is created by cloudformation when we specify the logrouter in CDK/CFN in the app container taskdef, the system adds that log-router sidecar container with the version set to latest automaticly.

Its possible to change that sidecar container image from latest to 2.19.0?

### Environment
- **CDK CLI Version:** 1.121.0 (build 026cb8f)
- **Module Version:** 1.121.0
- **Node.js Version:** 14
- **OS:** macOS
- **Language (Version):** TypeScript

### Other information
logging: ecs.LogDrivers.firelens({
options: {
Name: 'datadog',
provider: 'ecs',
TLS: 'on',
dd_message_key: 'log',
dd_service: 'test_service,
dd_source: 'java',
dd_tags: 'env:test',
},
secretOptions: {
apikey: ecs.Secret.fromSsmParameter(
REDACTED
),
},
}),

Thanks for your time!!

Start at ecs.LogDrivers.firelens and trace how the ECS task definition produces the log-router sidecar in CloudFormation. Determine whether the image should default to a stable version or be configurable, then verify that synthesized CloudFormation uses the selected image.

Written by the indexing model from the issue text.