aws-lambda-python-alpha: `PythonFunction` requires 4 minutes to build and produces a 3GB Docker image

### Describe the bug

The Docker image for this Lambda function was built on an AMD Ryzen 5 7600X CPU.

The resulting image size of 3GB is highly excessive, especially considering that the final Lambda function deployment package is only 3.6MB.




### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

See above.

### Current Behavior

See above.

### Reproduction Steps

See above.

### Possible Solution

Refactor the [Dockerfile](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-lambda-python-alpha/lib/Dockerfile).

### Additional Information/Context

Related issue: #32413

### CDK CLI Version

2.173.3

### Framework Version

_No response_

### Node.js Version

22.12.0

### OS

Ubuntu 24.04.1

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

The relevant entry point is packages/@aws-cdk/aws-lambda-python-alpha/lib/Dockerfile. Start by reading that Dockerfile and the related issue #32413, then compare its build output with the reported 3.6MB deployment package. Done means the PythonFunction build no longer takes about four minutes or produces a roughly 3GB image while preserving the deployment package.

Written by the indexing model from the issue text.