Feature request: Extend SAM container management to Bedrock AgentCore, ECS, and Fargate

## Description

AWS SAM currently handles container image management (build, package, push) for Lambda functions, but this capability does not extend to other container-based compute targets like Bedrock AgentCore, ECS, or Fargate.

When using Bedrock AgentCore's runtime, developers must manage the entire container lifecycle outside of SAM — building, tagging, and pushing images manually — then deploy the remaining infrastructure through SAM. This breaks the unified development workflow that SAM provides for Lambda.

## Proposed Solution

Extend SAM's container management capabilities (build, package, push) to support:

- **Bedrock AgentCore** runtime containers
- **ECS** task definition containers
- **Fargate** containers

SAM already knows how to manage container images end-to-end for Lambda. Applying the same pattern to other AWS compute services would keep the workflow consistent and significantly reduce friction for developers building multi-service applications.

## Additional Context

This is especially relevant as more developers adopt Bedrock AgentCore for agent-based workloads. Having to drop out of SAM to manage the container, then come back to SAM for the rest of the infrastructure, creates an unnecessary seam in the developer experience.

## SAM CLI Version

N/A — feature request

## Additional Environment Details

N/A