### 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_
parts = line.split()
                if len(parts) < 4:
                    
                    continue
                mount_point = parts[1]
                options = parts[3]
                
                if mount_point != "/":
                    continue
                if "io.containerd" in options:
                    return True
    except FileNotFoundError:
        pass
    return False
OVERRIDE_IN_DOCKER = parse_boolean_env("OVERRIDE_IN_DOCKER")
is_in_docker = in_docker()
is_in_linux = is_linux()
CONFIG_PROFILE = os.environ.get("CONFIG_PROFILE", "").strip()
CONFIG_DIR = os.environ.get("CONFIG_DIR", os.path.expanduser("~/.localstack"))
try:
    
    LOADED_PROFILE = load_environment(CONFIG_PROFILE)
except ImportError:
    
    LOADED_PROFILE = None
DEFAULT_REGION = (
    os.environ.get("DEFAULT_REGION") or os.environ.get("AWS_DEFAULT_REGION") or AWS_REGION_US_EAST_1
)
DATA_DIR = os.environ.get("DATA_DIR", "").strip()