### 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_
return str(self.__dict__)
def eval_log_type(env_var_name: str) -> Union[str, bool]:
    """Get the log type from environment variable"""
    ls_log = os.environ.get(env_var_name, "").lower().strip()
    return ls_log if ls_log in LOG_LEVELS else False
def parse_boolean_env(env_var_name: str) -> Optional[bool]:
    """Parse the value of the given env variable and return True/False, or None if it is not a boolean value."""
    value = os.environ.get(env_var_name, "").lower().strip()
    if value in TRUE_STRINGS:
        return True
    if value in FALSE_STRINGS:
        return False
    return None
def is_env_true(env_var_name: str) -> bool:
    """Whether the given environment variable has a truthy value."""
    return os.environ.get(env_var_name, "").lower().strip() in TRUE_STRINGS
def is_env_not_false(env_var_name: str) -> bool: