### 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_
),
                os.path.exists("/sys/fs/cgroup/docker/"),
                any(
                    "docker-" in file_names
                    for file_names in os.listdir("/sys/fs/cgroup/system.slice/")
                ),
            ]
        ):
            return False
    except Exception:
        pass
    with open("/proc/1/cgroup", "rt") as ifh:
        content = ifh.read()
        if "docker" in content or "buildkit" in content:
            return True
        os_hostname = socket.gethostname()
        if os_hostname and os_hostname in content:
            return True
    
    
    try:
        with open("/proc/mounts", "rt") as infile:
            for line in infile:
                line = line.strip()
                if not line:
                    continue
                
                if line[0] == "#":
                    continue