Feature request: cluster-wide ENV definitions

Use case: I'm running a mix of ECS, EC2, and AWS resources like RDS and ElastiCache. I have separate staging and production environments on different VPCs. Right now, to convey the connection URL for my ElastiCache redis instance, I have to set an environment variable REDIS_HOST on every single task definition that I create. It's different for staging vs. production, so this means I have to create different tasks for every container in every environment. This is manageable with automation, but also a bit overly complex.

Another use case would be setting RUBY_ENV or RAILS_ENV for every task.

Proposed solution: Add the ability to define env variables per ECS cluster, and have them applied to all containers run within that cluster.

No repository files, tests, or entry points are mentioned. Start by reviewing how ECS clusters and task definitions currently receive environment variables, then define the cluster-level behavior and verify that variables are applied to all containers launched in a cluster without requiring per-task definitions.

Written by the indexing model from the issue text.