### 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 self._provider_config.get(service, self.default_value)
    def set_provider_if_not_exists(self, service: str, provider: str) -> None:
        if service not in self._provider_config:
            self._provider_config[service] = provider
    def set_provider(self, service: str, provider: str):
        self._provider_config[service] = provider
    def bulk_set_provider_if_not_exists(self, services: List[str], provider: str):
        for service in services:
            self.set_provider_if_not_exists(service, provider)
    def __getitem__(self, item):
        return self.get_provider(item)
    def __setitem__(self, key, value):
        self.set_provider(key, value)
    def __len__(self):
        return len(self._provider_config)
    def __iter__(self):
        return self._provider_config.__iter__()
SERVICE_PROVIDER_CONFIG = ServiceProviderConfig("default")
SERVICE_PROVIDER_CONFIG.load_from_environment()
def init_directories() -> Directories:
    if is_in_docker:
        return Directories.for_container()
    else:
        if is_env_true("LOCALSTACK_CLI"):
            return Directories.for_cli()
        return Directories.for_host()
dirs: Directories
dirs = init_directories()