test(batch): cover ContainerImage.fromAsset ECR image assets

[sakul-learning]

Gap

The AWS Batch port accepts ecs.ContainerImage, and the shared API already exposes ContainerImage.fromAsset(). However, the equivalent Batch coverage remains commented out because the upstream test/batchjob-image/ Dockerfile fixture was not ported.

This leaves the local-Docker-context → staging → Docker build → ECR image URI → Batch containerProperties.image path unverified for Batch, even though the TerraConstructs implementation is intended to support it. fromRegistry() and fromEcrRepository() coverage do not exercise that path.

Related: #136 (Batch port). This is distinct from #124 / the pending CDKTN 0.24 asset-pipeline release: #124 changes the generic staging implementation, but does not add the ECR publishing behavior.

Scope

1. Port or add the Batch batchjob-image/ Docker build-context fixture (including its Dockerfile) from the relevant upstream AWS CDK test coverage.
2. Restore or replace the commented ContainerImage.fromAsset(path.join(__dirname, "batchjob-image")) synthesis test for an ECS Batch job definition.
3. Assert the synthesized Batch containerProperties.image is wired to the Docker image asset/ECR URI—not merely a registry literal—and retain assertions appropriate for the asset dependency.
4. Run a targeted live Batch deployment: build/publish the image asset, submit/run the Batch workload, verify successful completion/log output, and destroy the test resources. Capture the result in the PR.

Timing / dependency

If #124 lands, run this coverage against its CDKTN-backed staging path. #124 itself must wait for the CDKTN 0.24 release line; this issue tracks the missing Batch-specific fixture and evidence independently.

Acceptance criteria

• Version-controlled fixture exists and is used by a non-commented synthesis/integration test.
• The test exercises ecs.ContainerImage.fromAsset() for batch.EcsJobDefinition.
• The test proves the rendered Batch image is the generated ECR asset image URI.
• Targeted live deploy → Batch execution → verification → destroy evidence is attached to the implementing PR.