### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

There appears to be an issue with this variable `export LAMBDA_KEEPALIVE_MS=1337`.
It's not picked up when you start `localstack start -d`

```shell
$ export LAMBDA_KEEPALIVE_MS=1000
$ env | grep LAMBDA_KEEPALIVE_MS
LAMBDA_KEEPALIVE_MS=1000
$ localstack start -d
$ docker inspect localstack_main -f '{{json .Config.Env}}' | grep LAMBDA_KEEPALIVE_MS
```

### Expected Behavior

It should be set in the container environment.

### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    awslocal s3 mb s3://mybucket


### Environment

```markdown
- OS: Windows (WSL)
- LocalStack: 2.2.1.dev20230731233910
```


### Anything else?

_No response_
import logging
import os
import platform
import socket
import subprocess
import tempfile
import time
from dataclasses import dataclass
from typing import Any, Dict, List, Mapping, Optional, Tuple, TypeVar, Union
from localstack import constants
from localstack.constants import (
    AWS_REGION_US_EAST_1,
    DEFAULT_BUCKET_MARKER_LOCAL,
    DEFAULT_DEVELOP_PORT,
    DEFAULT_LAMBDA_CONTAINER_REGISTRY,
    DEFAULT_VOLUME_DIR,
    ENV_INTERNAL_TEST_COLLECT_METRIC,
    ENV_INTERNAL_TEST_RUN,
    FALSE_STRINGS,
    LOCALHOST,
    LOCALHOST_IP,
    LOCALSTACK_ROOT_FOLDER,
    LOG_LEVELS,