### 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_
environment: Dict[str, str]
) -> Tuple[str, str, str, int, int]:
    if is_in_docker:
        default_ip = "0.0.0.0"
    else:
        default_ip = "127.0.0.1"
    localstack_host_raw = environment.get("LOCALSTACK_HOST")
    gateway_listen_raw = environment.get("GATEWAY_LISTEN")
    
    
    localstack_host = localstack_host_raw
    if localstack_host is None:
        localstack_host = f"{constants.LOCALHOST_HOSTNAME}:{constants.DEFAULT_PORT_EDGE}"
    else:
        localstack_host = parse_hostname_and_ip(
            localstack_host,
            default_host=constants.LOCALHOST_HOSTNAME,
        )
    gateway_listen = gateway_listen_raw
    if gateway_listen is None:
        
        try:
            port = int(localstack_host.split(":", 1)[-1])
        except ValueError:
            port = constants.DEFAULT_PORT_EDGE
        gateway_listen = f"{default_ip}:{port}"
    else: