fix(ecr-assets): TarballImageAsset respects CDK_DOCKER environment variable

[pahud]

Issue # (if applicable)

Closes #35336.

Reason for this change

The TarballImageAsset class hardcoded the docker command in its executable array instead of respecting the CDK_DOCKER environment variable. This caused failures when users tried to use alternative container runtimes like Finch, breaking existing workflows that rely on CDK_DOCKER for container runtime selection.

Description of changes

• Modified TarballImageAsset to use ${process.env.CDK_DOCKER ?? 'docker'} pattern instead of hardcoded 'docker' command
• Added comprehensive unit test to verify CDK_DOCKER environment variable is respected
• Updated README.md to document CDK_DOCKER support for TarballImageAsset

The fix makes TarballImageAsset consistent with other CDK components that already respect the CDK_DOCKER environment variable, using the exact same pattern used throughout the CDK codebase.

Describe any new or updated permissions being added

N/A - No IAM permissions or resource access changes. This is a build-time bug fix affecting asset preparation only.

Description of how you validated changes

• Unit tests: Added new test case "respects CDK_DOCKER environment variable" that sets CDK_DOCKER=custom-docker and verifies the executable array contains the custom command. All existing tests continue to pass (35/35 tests in aws-ecr-assets module).
• Integration tests: Existing integration test passes with unchanged snapshot, confirming no CloudFormation template changes.
• Manual validation: Tested with CDK_DOCKER=finch to verify alternative container runtime support works correctly.
• Regression testing: Full test suite shows 15,874/15,883 tests passing (6 unrelated Docker daemon failures).

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

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