### 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_DEV_PORT_EXPOSE",
    "LAMBDA_RUNTIME_EXECUTOR",
    "LAMBDA_RUNTIME_ENVIRONMENT_TIMEOUT",
    "LAMBDA_STAY_OPEN_MODE",
    "LAMBDA_TRUNCATE_STDOUT",
    "LAMBDA_RETRY_BASE_DELAY_SECONDS",
    "LAMBDA_SYNCHRONOUS_CREATE",
    "LAMBDA_LIMITS_CONCURRENT_EXECUTIONS",
    "LAMBDA_LIMITS_MINIMUM_UNRESERVED_CONCURRENCY",
    "LAMBDA_LIMITS_TOTAL_CODE_SIZE",
    "LAMBDA_LIMITS_CODE_SIZE_ZIPPED",
    "LAMBDA_LIMITS_CODE_SIZE_UNZIPPED",
    "LAMBDA_LIMITS_CREATE_FUNCTION_REQUEST_SIZE",
    "LAMBDA_LIMITS_MAX_FUNCTION_ENVVAR_SIZE_BYTES",
    "LEGACY_DIRECTORIES",
    "LEGACY_DOCKER_CLIENT",
    "LEGACY_EDGE_PROXY",
    "LEGACY_SNS_GCM_PUBLISHING",
    "LOCALSTACK_API_KEY",
    "LOCALSTACK_AUTH_TOKEN",
    "LOCALSTACK_HOST",
    "LOCALSTACK_HOSTNAME",
    "LOG_LICENSE_ISSUES",
    "LS_LOG",
    "MAIN_CONTAINER_NAME",
    "MAIN_DOCKER_NETWORK",
    "OPENSEARCH_ENDPOINT_STRATEGY",
    "OUTBOUND_HTTP_PROXY",
    "OUTBOUND_HTTPS_PROXY",
    "PARITY_AWS_ACCESS_KEY_ID",