### 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_
value: str, default_host: str, default_port: int = constants.DEFAULT_PORT_EDGE
) -> str:
    """
    Given a string that should contain a <hostname>:<port>, if either are
    absent then use the defaults.
    """
    host, port = default_host, default_port
    if ":" in value:
        hostname, port_s = value.split(":", 1)
        if hostname.strip():
            host = hostname.strip()
        try:
            port = int(port_s)
        except (ValueError, TypeError):
            pass
    else:
        if value.strip():
            host = value.strip()
    return f"{host}:{port}"
def populate_legacy_edge_configuration(