Feat. Request: Allow environment variables to depend on CF intrinsic functions

[kazkansouh]

Context

When trying to reference items generated in the CloudForamtion script its often useful to pass the ARN to a docker container as an environment variable.

However, currently its not possible to directly use the CloudFormation intrinsic functions (e.g. !Ref) within the service definition. E.g.

services:
  server:
    image: someimage
    environment:
      TESTX:
        Fn::GetAtt: [ x, y ]

There are often times this could be useful, e.g. accessing the CloudMap object.

Alternatives

A workaround to this is really inconvenient as it relies on using the Resources block to overwrite the majority of the task definition. For example, consider the below excerpt of a simple task definition obtained with docker compose convert. I would need to completely overwrite the whole of the ContainerDefinitions list, just to modify the environment variable defined within it.

  ServerTaskDefinition:
    Properties:
      ContainerDefinitions:
      - Command:
        - eu-west-1.compute.internal
        - simple-server.local
        Essential: false
        Image: docker/ecs-searchdomain-sidecar:1.0
        LogConfiguration:
          LogDriver: awslogs
          Options:
            awslogs-group:
              Ref: LogGroup
            awslogs-region:
              Ref: AWS::Region
            awslogs-stream-prefix: simple-server
        Name: Server1_ResolvConf_InitContainer
      - DependsOn:
        - Condition: SUCCESS
          ContainerName: Server1_ResolvConf_InitContainer
        Environment:
        - Name: TESTX
          Value: testx
        Essential: true
        Image: someimage
        LinuxParameters: {}
        LogConfiguration:
          LogDriver: awslogs
          Options:
            awslogs-group:
              Ref: LogGroup
            awslogs-region:
              Ref: AWS::Region
            awslogs-stream-prefix: simple-server
        Name: server
        PortMappings:
        - ContainerPort: 80
          HostPort: 80
          Protocol: tcp
      Cpu: "256"
      ExecutionRoleArn:
        Ref: ServerTaskExecutionRole
      Family: simple-server-server
      Memory: "512"
      NetworkMode: awsvpc
      RequiresCompatibilities:
      - FARGATE
    Type: AWS::ECS::TaskDefinition

I assume allowing intrinsic functions in the service definition would be considered bad as it would prevent it being used outside of ECS. Perhaps the best is to support a way to merge changes into the ContainerDefinitions list. For example:

services:
  server:
    image: someimage
    environment:
      TESTX: testx

x-aws-cloudformation:
  Resources:
    ServerTaskDefinition:
      Properties:
        ContainerDefinitions:
          -
          - Environment:
              - Name: TESTX
                 Value: 
                   Fn::GetAtt: [ x, y ]

Currently, the above definition will be non-functional as the ContainerDefinitions will be corrupted.

Additional context

$ docker compose version
Docker Compose version 1.0.17

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

This is a 🚀 Feature Request