docs(ecs): document Podman as a Docker/Finch alternative

[sAchin-680]

docs(aws-ecs): Document Podman as a Docker/Finch alternative

• Add Podman configuration examples in aws-ecr-assets README
• Document DOCKER_HOST requirement for both Finch and Podman
• Update CONTRIBUTING.md with Podman setup instructions
• Provide community-tested alternative guidance for Docker licensing concerns

Fixes #35833

Issue # (if applicable)

Closes #35833.

Reason for this change

Users are facing Docker licensing issues and seeking alternatives like Podman for CDK container builds. The current documentation only mentions Finch as a Docker alternative but lacks:

1. Podman configuration examples despite it being a popular, lightweight Docker alternative
2. Critical DOCKER_HOST environment variable requirements for both Finch and Podman
3. Clear guidance on community-tested alternatives for users with Docker licensing concerns

This creates a documentation gap that prevents users from successfully using Podman with CDK, even though it works with proper configuration.

Description of changes

1. Enhanced aws-ecr-assets README.md:

• Added new "Docker Alternatives" section with structured examples
• Documented exact Podman configuration: CDK_DOCKER=podman and DOCKER_HOST=$(podman machine inspect...)
• Added missing DOCKER_HOST configuration for Finch
• Included clear support level distinctions (AWS-supported vs community-tested)
• Added explanatory note about DOCKER_HOST requirement for container daemon communication

2. Updated CONTRIBUTING.md:

• Extended Docker replacement documentation to include Podman
• Added DOCKER_HOST requirement explanation for both Finch and Podman
• Provided specific configuration example for Podman setup

3. Design decisions:

• Used the exact configuration validated by the issue reporter (@TrevorBurnham)
• Maintained clear distinction between official AWS support (Finch) and community-tested alternatives (Podman)
• Structured documentation to be copy-paste friendly for users
• Positioned content logically after existing CDK_DOCKER explanation

4. Alternatives considered:

• Could have only documented Podman without DOCKER_HOST - rejected because it's critical for functionality
• Could have provided minimal examples - rejected in favor of complete, working configurations
• Could have avoided mentioning support levels - rejected because transparency is important

Describe any new or updated permissions being added

No new or updated IAM permissions are being added. This is purely documentation enhancement.

Description of how you validated changes

Documentation validation:

• Verified configuration examples match the working setup from issue (aws-ecs): Document Podman as a Docker/Finch alternative #35833
• Confirmed both CONTRIBUTING.md and aws-ecr-assets README.md are updated consistently
• Ensured documentation follows existing patterns and formatting
• Cross-referenced with existing Finch documentation for consistency
• Validated markdown formatting and code block syntax

Content validation:

• Examples use the exact environment variables validated by the community
• Support level messaging aligns with AWS policy (official support for Finch, community-tested for Podman)
• Documentation addresses all points raised in the original issue

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

(This review is outdated)

[sAchin-680]

Hi @pahud, @aws/aws-cdk-maintainers, and code owners

When you have a moment, could you please review this PR?

This update enhances the ECS documentation by adding Podman as a community-tested alternative to Docker/Finch, addressing configuration and setup gaps reported by users. It’s a documentation-only change aimed at improving developer experience when using CDK container builds without Docker licensing constraints.

Your feedback and review would be greatly appreciated.

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.