### 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_
global CONFIG_ENV_VARS
    CONFIG_ENV_VARS += [
        key
        for key in [key.upper() for key in os.environ]
        if key.startswith("LOCALSTACK_") or key.startswith("PROVIDER_OVERRIDE_")
    ]
    
    CONFIG_ENV_VARS += [
        "LOCALSTACK_" + v for v in CONFIG_ENV_VARS if not v.startswith("LOCALSTACK_")
    ]
    CONFIG_ENV_VARS = list(set(CONFIG_ENV_VARS))
populate_config_env_var_names()
def service_port(service_key: str, external: bool = False) -> int:
    service_key = service_key.lower()
    if external:
        if service_key == "sqs" and SQS_PORT_EXTERNAL:
            return SQS_PORT_EXTERNAL
    return get_edge_port_http()
def get_protocol():
    return "https" if USE_SSL else "http"
def service_url(service_key, host=None, port=None):
    host = host or LOCALHOST
    port = port or service_port(service_key)
    return f"{get_protocol()}://{host}:{port}"
def external_service_url(service_key, host=None, port=None):
    host = host or HOSTNAME_EXTERNAL
    port = port or service_port(service_key, external=True)
    return service_url(service_key, host=host, port=port)
def get_edge_port_http():