DockerImageAsset: environmental variables

### Describe the feature

Is it possible to include environmental variables in the docker build? I don't see it included here: https://docs.aws.amazon.com/cdk/api/v2/python/aws_cdk.aws_ecr_assets/DockerImageAsset.html

### Use Case

I would like to include the following env vars when building docker images with CDK:
```
DOCKER_BUILDKIT=1
CDK_DOCKER=finch
```

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

latest

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

MacOS

The issue points to the DockerImageAsset API documentation; start there, then locate its implementation and existing Docker build tests. Define how environment variables are passed during the build and verify the behavior with focused tests and the API documentation; no target files are named in the issue.

Written by the indexing model from the issue text.