### 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_
if LS_LOG:
        log_level = str(LS_LOG).upper()
        return log_level.lower() in TRACE_LOG_LEVELS
    return False
if DEBUG:
    logging.getLogger("").setLevel(logging.DEBUG)
    logging.getLogger("localstack").setLevel(logging.DEBUG)
LOG = logging.getLogger(__name__)
if is_trace_logging_enabled():
    load_end_time = time.time()
    LOG.debug(
        "Initializing the configuration took %s ms", int((load_end_time - load_start_time) * 1000)
    )
HOSTNAME_EXTERNAL = os.environ.get("HOSTNAME_EXTERNAL", "").strip() or LOCALHOST
LOCALSTACK_HOSTNAME = os.environ.get("LOCALSTACK_HOSTNAME", "").strip() or LOCALHOST
def parse_hostname_and_ip(