### 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_
DOCKER_BRIDGE_IP = "172.17.0.1"
    if is_in_docker:
        candidates = (DOCKER_BRIDGE_IP, "172.18.0.1")
        for ip in candidates:
            if ping(ip):
                DOCKER_BRIDGE_IP = ip
                break
try:
    DOCKER_HOST_FROM_CONTAINER = DOCKER_BRIDGE_IP
    if not is_in_docker and not is_in_linux:
        
        
        if LOCALSTACK_HOSTNAME == LOCALHOST:
            DOCKER_HOST_FROM_CONTAINER = "host.docker.internal"
    
    if is_in_docker:
        try:
            DOCKER_HOST_FROM_CONTAINER = socket.gethostbyname("host.docker.internal")
        except socket.error:
            DOCKER_HOST_FROM_CONTAINER = socket.gethostbyname("host.containers.internal")
        if LOCALSTACK_HOSTNAME == DOCKER_BRIDGE_IP:
            LOCALSTACK_HOSTNAME = DOCKER_HOST_FROM_CONTAINER
except socket.error:
    pass
EXTERNAL_SERVICE_PORTS_START = int(
    os.environ.get("EXTERNAL_SERVICE_PORTS_START")
    or os.environ.get("SERVICE_INSTANCES_PORTS_START")
    or 4510
)
EXTERNAL_SERVICE_PORTS_END = int(