### 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_
LAMBDA_LIMITS_CODE_SIZE_ZIPPED = int(os.environ.get("LAMBDA_LIMITS_CODE_SIZE_ZIPPED", 52_428_800))
LAMBDA_LIMITS_CODE_SIZE_UNZIPPED = int(
    os.environ.get("LAMBDA_LIMITS_CODE_SIZE_UNZIPPED", 262_144_000)
)
LAMBDA_LIMITS_CREATE_FUNCTION_REQUEST_SIZE = int(
    os.environ.get("LAMBDA_LIMITS_CREATE_FUNCTION_REQUEST_SIZE", 69_905_067)
)
LAMBDA_LIMITS_MAX_FUNCTION_ENVVAR_SIZE_BYTES = int(
    os.environ.get("LAMBDA_LIMITS_MAX_FUNCTION_ENVVAR_SIZE_BYTES", 4 * 1024)
)
LAMBDA_DEV_PORT_EXPOSE = is_env_true("LAMBDA_DEV_PORT_EXPOSE")
LAMBDA_INIT_RELEASE_VERSION = os.environ.get("LAMBDA_INIT_RELEASE_VERSION")
LAMBDA_INIT_DEBUG = is_env_true("LAMBDA_INIT_DEBUG")
LAMBDA_INIT_BIN_PATH = os.environ.get("LAMBDA_INIT_BIN_PATH")
LAMBDA_INIT_BOOTSTRAP_PATH = os.environ.get("LAMBDA_INIT_BOOTSTRAP_PATH")
LAMBDA_INIT_DELVE_PATH = os.environ.get("LAMBDA_INIT_DELVE_PATH")
LAMBDA_INIT_DELVE_PORT = int(os.environ.get("LAMBDA_INIT_DELVE_PORT") or 40000)
LAMBDA_INIT_POST_INVOKE_WAIT_MS = os.environ.get("LAMBDA_INIT_POST_INVOKE_WAIT_MS")
LAMBDA_INIT_USER = os.environ.get("LAMBDA_INIT_USER")
LOCAL_PORT_STEPFUNCTIONS = int(os.environ.get("LOCAL_PORT_STEPFUNCTIONS") or 8083)
STEPFUNCTIONS_LAMBDA_ENDPOINT = os.environ.get("STEPFUNCTIONS_LAMBDA_ENDPOINT", "").strip()
WINDOWS_DOCKER_MOUNT_PREFIX = os.environ.get("WINDOWS_DOCKER_MOUNT_PREFIX", "/host_mnt")
S3_SKIP_SIGNATURE_VALIDATION = is_env_not_false("S3_SKIP_SIGNATURE_VALIDATION")
S3_SKIP_KMS_KEY_VALIDATION = is_env_not_false("S3_SKIP_KMS_KEY_VALIDATION")
LAMBDA_EXECUTOR = os.environ.get("LAMBDA_EXECUTOR", "").strip()
LAMBDA_FALLBACK_URL = os.environ.get("LAMBDA_FALLBACK_URL", "").strip()
LAMBDA_FORWARD_URL = os.environ.get("LAMBDA_FORWARD_URL", "").strip()
LAMBDA_CODE_EXTRACT_TIME = int(os.environ.get("LAMBDA_CODE_EXTRACT_TIME") or 25)
LAMBDA_STAY_OPEN_MODE = is_in_docker and is_env_not_false("LAMBDA_STAY_OPEN_MODE")
LAMBDA_TRUNCATE_STDOUT = int(os.getenv("LAMBDA_TRUNCATE_STDOUT") or 2000)