### 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_
)
HOSTNAME_FROM_LAMBDA = os.environ.get("HOSTNAME_FROM_LAMBDA", "").strip()
LAMBDA_REMOTE_DOCKER = is_env_true("LAMBDA_REMOTE_DOCKER")
if is_in_docker and not os.environ.get("LAMBDA_REMOTE_DOCKER", "").strip():
    LAMBDA_REMOTE_DOCKER = True
BUCKET_MARKER_LOCAL = (
    os.environ.get("BUCKET_MARKER_LOCAL", "").strip() or DEFAULT_BUCKET_MARKER_LOCAL
)
LAMBDA_DOCKER_NETWORK = os.environ.get("LAMBDA_DOCKER_NETWORK", "").strip()
LAMBDA_DOCKER_DNS = os.environ.get("LAMBDA_DOCKER_DNS", "").strip()
LAMBDA_DOCKER_FLAGS = os.environ.get("LAMBDA_DOCKER_FLAGS", "").strip()
LAMBDA_IGNORE_ARCHITECTURE = is_env_true("LAMBDA_IGNORE_ARCHITECTURE")
LAMBDA_PREBUILD_IMAGES = is_env_true("LAMBDA_PREBUILD_IMAGES")
LAMBDA_RUNTIME_EXECUTOR = os.environ.get("LAMBDA_RUNTIME_EXECUTOR", "").strip()
LAMBDA_RUNTIME_ENVIRONMENT_TIMEOUT = int(os.environ.get("LAMBDA_RUNTIME_ENVIRONMENT_TIMEOUT") or 10)
LAMBDA_CONTAINER_REGISTRY = (
    os.environ.get("LAMBDA_CONTAINER_REGISTRY", "").strip() or DEFAULT_LAMBDA_CONTAINER_REGISTRY
)
LAMBDA_RUNTIME_IMAGE_MAPPING = os.environ.get("LAMBDA_RUNTIME_IMAGE_MAPPING", "").strip()
LAMBDA_REMOVE_CONTAINERS = (
    os.environ.get("LAMBDA_REMOVE_CONTAINERS", "").lower().strip() not in FALSE_STRINGS
)
LAMBDA_KEEPALIVE_MS = int(os.environ.get("LAMBDA_KEEPALIVE_MS", 600_000))
LAMBDA_LIMITS_CONCURRENT_EXECUTIONS = int(
    os.environ.get("LAMBDA_LIMITS_CONCURRENT_EXECUTIONS", 1_000)
)
LAMBDA_LIMITS_MINIMUM_UNRESERVED_CONCURRENCY = int(
    os.environ.get("LAMBDA_LIMITS_MINIMUM_UNRESERVED_CONCURRENCY", 100)
)
LAMBDA_LIMITS_TOTAL_CODE_SIZE = int(os.environ.get("LAMBDA_LIMITS_TOTAL_CODE_SIZE", 80_530_636_800))