### 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_
"CI",
    "CUSTOM_SSL_CERT_PATH",
    "DEBUG",
    "DEBUG_HANDLER_CHAIN",
    "DEFAULT_REGION",
    "DEVELOP",
    "DEVELOP_PORT",
    "DISABLE_CORS_CHECKS",
    "DISABLE_CORS_HEADERS",
    "DISABLE_CUSTOM_CORS_APIGATEWAY",
    "DISABLE_CUSTOM_CORS_S3",
    "DISABLE_EVENTS",
    "DOCKER_BRIDGE_IP",
    "DOCKER_SDK_DEFAULT_TIMEOUT_SECONDS",
    "DYNAMODB_ERROR_PROBABILITY",
    "DYNAMODB_HEAP_SIZE",
    "DYNAMODB_IN_MEMORY",
    "DYNAMODB_SHARE_DB",
    "DYNAMODB_READ_ERROR_PROBABILITY",
    "DYNAMODB_WRITE_ERROR_PROBABILITY",
    "EAGER_SERVICE_LOADING",
    "EDGE_BIND_HOST",
    "EDGE_FORWARD_URL",
    "EDGE_PORT",
    "EDGE_PORT_HTTP",
    "ENABLE_CONFIG_UPDATES",
    "ES_CUSTOM_BACKEND",
    "ES_ENDPOINT_STRATEGY",
    "ES_MULTI_CLUSTER",
    "EXTRA_CORS_ALLOWED_HEADERS",