cdk-assets: private ecr registry support overwrite any additional docker configuration

### Describe the bug

- We have a proxy configuration for docker in the ./docker/config.json file.
- We are using private ecr to fetch our images which configuration is placed into the ~/.cdk/cdk-docker-creds.json file

The cdk-assets creates a new docker config file but it is only placing the contents of the ~/.cdk/cdk-docker-creds.json file and only the `credHelpers` property, so it is not even possible to include the proxy settings.

### Expected Behavior

Both configuration should be respected.

### Current Behavior

Only credentials are kept.

### Reproduction Steps

see before.

### Possible Solution

Change this line: https://github.com/aws/aws-cdk/blob/1fcef6309e54d7ec1e179c22a47a235b38fa1673/packages/cdk-assets/lib/private/docker.ts#L168C81-L168C92

Options:
1. Write out the whole ~/.cdk/cdk-docker-creds.json file not just the credHelpers
2. Add support to include additional ~/.cdk/cdk-docker*.json files to plug in additional docker configurations
3. Check whenever a ~./docker/config.json file exists and merge both files.

### Additional Information/Context

_No response_

### CDK CLI Version

2.147.0

### Framework Version

_No response_

### Node.js Version

Any

### OS

Any

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start with packages/cdk-assets/lib/private/docker.ts at the linked line and trace how the temporary Docker configuration is assembled from cdk-docker-creds.json. Reproduce the setup with proxy settings in ./docker/config.json, then verify that the resulting configuration preserves both proxy settings and ECR credential helpers.

Written by the indexing model from the issue text.