### 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_
"EXTRA_CORS_ALLOWED_ORIGINS",
    "EXTRA_CORS_EXPOSE_HEADERS",
    "GATEWAY_LISTEN",
    "HOSTNAME",
    "HOSTNAME_EXTERNAL",
    "HOSTNAME_FROM_LAMBDA",
    "KINESIS_ERROR_PROBABILITY",
    "KINESIS_INITIALIZE_STREAMS",
    "KINESIS_MOCK_PERSIST_INTERVAL",
    "KINESIS_ON_DEMAND_STREAM_COUNT_LIMIT",
    "LAMBDA_CODE_EXTRACT_TIME",
    "LAMBDA_CONTAINER_REGISTRY",
    "LAMBDA_DOCKER_DNS",
    "LAMBDA_DOCKER_FLAGS",
    "LAMBDA_DOCKER_NETWORK",
    "LAMBDA_EXECUTOR",
    "LAMBDA_FALLBACK_URL",
    "LAMBDA_FORWARD_URL",
    "LAMBDA_INIT_DEBUG",
    "LAMBDA_INIT_BIN_PATH",
    "LAMBDA_INIT_BOOTSTRAP_PATH",
    "LAMBDA_INIT_DELVE_PATH",
    "LAMBDA_INIT_DELVE_PORT",
    "LAMBDA_INIT_POST_INVOKE_WAIT",
    "LAMBDA_INIT_USER",
    "LAMBDA_INIT_RELEASE_VERSION",
    "LAMBDA_RUNTIME_IMAGE_MAPPING",
    "LAMBDA_JAVA_OPTS",
    "LAMBDA_REMOTE_DOCKER",
    "LAMBDA_REMOVE_CONTAINERS",