(aws-lambda): Expose ImagePath from DockerBuildAssetOptions in Container as ENV

### Describe the feature

As a Quality of Life improvement, exposing the chosen, or default, path to the container building, would make it easier to work with on build scripts

### Use Case

When working with different build scripts, it woul be good to know from the container/script side where to put the final assets

### Proposed Solution

Add a `CDK_ASSET_IMAGE_PATH` environment variable when building the asset

### Other Information

_No response_

### Acknowledgements

- [X] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.67.0

### Environment details (OS name and version, etc.)

Windows 10

Start by tracing DockerBuildAssetOptions and the Container asset-building path in the AWS CDK repository. Confirm how the chosen or default image path reaches the container build, then ensure CDK_ASSET_IMAGE_PATH is available during the build with that value; done means build scripts can read the variable consistently.

Written by the indexing model from the issue text.