### 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 running in a docker container, else False
    Ref. https://docs.docker.com/config/containers/runmetrics/#control-groups
    """
    if OVERRIDE_IN_DOCKER is not None:
        return OVERRIDE_IN_DOCKER
    
    for path in [
        "/usr/lib/localstack/.community-version",
        "/usr/lib/localstack/.pro-version",
        "/tmp/localstack/.marker",
    ]:
        if os.path.isfile(path):
            return True
    
    if os.path.exists("/.dockerenv"):
        return True
    if os.path.exists("/run/.containerenv"):
        return True
    if not os.path.exists("/proc/1/cgroup"):
        return False
    try:
        if any(
            [
                os.path.exists("/sys/fs/cgroup/memory/docker/"),
                any(
                    "docker-" in file_names
                    for file_names in os.listdir("/sys/fs/cgroup/memory/system.slice")