### 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_
"PERSISTENCE",
    "PORTS_CHECK_DOCKER_IMAGE",
    "REQUESTS_CA_BUNDLE",
    "S3_SKIP_SIGNATURE_VALIDATION",
    "S3_SKIP_KMS_KEY_VALIDATION",
    "SERVICES",
    "SKIP_INFRA_DOWNLOADS",
    "SKIP_SSL_CERT_DOWNLOAD",
    "SNAPSHOT_LOAD_STRATEGY",
    "SNAPSHOT_SAVE_STRATEGY",
    "SNAPSHOT_FLUSH_INTERVAL",
    "SQS_DELAY_PURGE_RETRY",
    "SQS_DELAY_RECENTLY_DELETED",
    "SQS_ENDPOINT_STRATEGY",
    "SQS_PORT_EXTERNAL",
    "SQS_DISABLE_CLOUDWATCH_METRICS",
    "SQS_CLOUDWATCH_METRICS_REPORT_INTERVAL",
    "STEPFUNCTIONS_LAMBDA_ENDPOINT",
    "SYNCHRONOUS_KINESIS_EVENTS",
    "SYNCHRONOUS_SNS_EVENTS",
    "TEST_AWS_ACCOUNT_ID",
    "TEST_IAM_USER_ID",
    "TEST_IAM_USER_NAME",
    "TF_COMPAT_MODE",
    "USE_SINGLE_REGION",
    "USE_SSL",
    "WAIT_FOR_DEBUGGER",
    "WINDOWS_DOCKER_MOUNT_PREFIX",
]
def is_local_test_mode() -> bool: