### 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_
components = gateway_listen.split(",")
        if len(components) > 1:
            LOG.warning("multiple GATEWAY_LISTEN addresses are not currently supported")
        gateway_listen = ",".join(
            [
                parse_hostname_and_ip(
                    component.strip(),
                    default_host=default_ip,
                )
                for component in components
            ]
        )
    assert gateway_listen is not None
    assert localstack_host is not None
    def legacy_fallback(envar_name: str, default: T) -> T:
        result = default
        result_raw = environment.get(envar_name)
        if result_raw is not None and gateway_listen_raw is None:
            result = result_raw
        return result
    
    
    edge_bind_host = legacy_fallback("EDGE_BIND_HOST", get_gateway_listen(gateway_listen)[0].host)
    edge_port = int(legacy_fallback("EDGE_PORT", get_gateway_listen(gateway_listen)[0].port))
    edge_port_http = int(
        legacy_fallback("EDGE_PORT_HTTP", 0),
    )
    return localstack_host, gateway_listen, edge_bind_host, edge_port, edge_port_http
@dataclass
class HostAndPort: