(aws_lambda_python_alpha): Docker build happens at import time rather than synth/deploy time

### Describe the bug

I have a python cdk codebase

I added a `aws_lambda_python_alpha.PythonFunction` to my stack... now as soon as I _instantiate_ the stack containing that Lambda, a docker build gets triggered

### Expected Behavior

I would expect that docker build for the Lambda function would only occur during deployment, or at worst during synth phase

docker build at import time feels like a bug

### Current Behavior

the docker build occurs at the point that the `aws_lambda_python_alpha.PythonFunction` gets instantiated

so it happens just by importing the code

in particular it happens when I run unit tests for the cdk codebase, which instantiate a version of the stack multiple times

a) makes my tests very slow 😢
b) it's also noisy - it prints a bunch of output to stderr:

```
#0 building with "desktop-linux" instance using docker driver

#1 [internal] load .dockerignore
#1 transferring context: 2B done
#1 DONE 0.0s

#2 [internal] load build definition from Dockerfile
#2 transferring dockerfile: 1.35kB done
#2 DONE 0.0s

#3 [internal] load metadata for public.ecr.aws/sam/build-python3.11:latest
#3 DONE 0.2s

#4 [1/2] FROM public.ecr.aws/sam/build-python3.11@sha256:1468e6000e3d406ab5b32e82ba3358440faf75398eef30aa997e2c8e4ad85b07
#4 DONE 0.0s

#5 [2/2] RUN python -m venv /usr/app/venv && mkdir /tmp/pip-cache && chmod -R 777 /tmp/pip-cache && pip install --upgrade pip && mkdir /tmp/poetry-cache && chmod -R 777 /tmp/poetry-cache && pip install pipenv==2022.4.8 poetry==1.5.1 && rm -rf /tmp/pip-cache/* /tmp/poetry-cache/*
#5 CACHED

#6 exporting to image
#6 exporting layers done
#6 writing image sha256:e1b922db4e93213d2e2d81648a4527a255153724fb3bee8c5d60b52414d89c12 done
#6 naming to docker.io/library/cdk-db7ab25e39b5606291af96999b213f4ca620fbee93139bb8d158e76572cf1431 done
#6 DONE 0.0s

What's Next?
View a summary of image vulnerabilities and recommendations → docker scout quickview
Bundling asset Website/Repro Lambda/Code/Stage...
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
sending incremental file list
index.py
requirements.txt

sent 227 bytes received 54 bytes 562.00 bytes/sec
total size is 45 speedup is 0.16
```

### Reproduction Steps

`app.py`
```python
from aws_cdk import (
App,
Environment,
Stack,
aws_lambda,
aws_lambda_python_alpha as python_lambda,
)
from constructs import Construct

class Website(Stack):
def __init__(
self,
scope: Construct,
id: str,
**kwargs,
):
super().__init__(scope, id)

# if I drop into debugger from tests, docker build occurs here:
python_lambda.PythonFunction(
self,
"Repro Lambda",
entry="infra/assets/repro/src",
runtime=aws_lambda.Runtime.PYTHON_3_11,
)

app = App()

Website(
app,
"Website",
env=Environment(
account="000000000000", # or a valid account
region="eu-west-1",
),
)

if __name__ == "__main__":
app.synth()
```

`infra/assets/repro/src/index.py`
```python
def handler(event, context):
return True
```

`infra/assets/repro/src/requirements.txt` (empty file)

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.106.1 (build a2e5f65)

### Framework Version

2.108.0

### Node.js Version

v18.18.0

### OS

macOS 14.1

### Language

Python

### Language Version

3.11.5

### Other information

_No response_

The reproduction is in app.py and uses aws_lambda_python_alpha.PythonFunction with assets under infra/assets/repro/src. Trace the PythonFunction construction and asset bundling path to find why Docker runs during stack instantiation rather than synth or deploy. Run the unit tests that instantiate the stack repeatedly; done means import and instantiation no longer trigger Docker while bundling still works at the intended lifecycle stage.

Written by the indexing model from the issue text.