### 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_
"""Returns True if we are running in the context of our local integration tests."""
    return is_env_true(ENV_INTERNAL_TEST_RUN)
def is_collect_metrics_mode() -> bool:
    """Returns True if metric collection is enabled."""
    return is_env_true(ENV_INTERNAL_TEST_COLLECT_METRIC)
def collect_config_items() -> List[Tuple[str, Any]]:
    """Returns a list of key-value tuples of LocalStack configuration values."""
    none = object()  
    
    keys = []
    keys.extend(CONFIG_ENV_VARS)
    keys.append("DATA_DIR")
    keys.sort()
    values = globals()
    result = []
    for k in keys:
        v = values.get(k, none)
        if v is none:
            continue
        result.append((k, v))
    result.sort()
    return result
def populate_config_env_var_names():