feature request: add host-local environment file to task container

### Summary

We have a desire to add host specific configuration to task containers, ideally via environment variables

### Description

We see that ecs task definitions support environment files, but currently only of type s3

Ideally a simple ecs agent config option to append an entry to all task containers environment files

### Expected Behavior

### Observed Behavior

### Environment Details

ECS anywhere

### Supporting Log Snippets

Start by reviewing ECS task-definition environment file support and the ECS Anywhere agent configuration described in the issue. Define how a host-local environment file would be appended to every task container and verify that the behavior is documented and consistently applied across ECS Anywhere task launches.

Written by the indexing model from the issue text.