### 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_
"""Whether the given environment variable is empty or has a truthy value."""
    return os.environ.get(env_var_name, "").lower().strip() not in FALSE_STRINGS
def load_environment(profile: str = None) -> Optional[str]:
    """Loads the environment variables from ~/.localstack/{profile}.env
    :param profile: the profile to load (defaults to "default")
    :returns str: the name of the actually loaded profile (might be the fallback)
    """
    if not profile:
        profile = "default"
    path = os.path.join(CONFIG_DIR, f"{profile}.env")
    if not os.path.exists(path):
        return None
    import dotenv
    dotenv.load_dotenv(path, override=False)
    return profile
def is_persistence_enabled() -> bool:
    return PERSISTENCE and dirs.data
def is_linux() -> bool:
    return platform.system() == "Linux"
def ping(host):
    """Returns True if host responds to a ping request"""
    is_windows = platform.system().lower() == "windows"
    ping_opts = "-n 1 -w 2000" if is_windows else "-c 1 -W 2"
    args = "ping %s %s" % (ping_opts, host)
    return (
        subprocess.call(args, shell=not is_windows, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        == 0
    )
def in_docker():