### 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_RETRY_BASE_DELAY_SECONDS = int(os.getenv("LAMBDA_RETRY_BASE_DELAY") or 60)
LAMBDA_SYNCHRONOUS_CREATE = is_env_true("LAMBDA_SYNCHRONOUS_CREATE")
KINESIS_INITIALIZE_STREAMS = os.environ.get("KINESIS_INITIALIZE_STREAMS", "").strip()
KMS_PROVIDER = (os.environ.get("KMS_PROVIDER") or "").strip() or "moto"
OPENSEARCH_CUSTOM_BACKEND = (
    os.environ.get("OPENSEARCH_CUSTOM_BACKEND", "").strip()
    or os.environ.get("ES_CUSTOM_BACKEND", "").strip()
)
OPENSEARCH_ENDPOINT_STRATEGY = (
    os.environ.get("OPENSEARCH_ENDPOINT_STRATEGY", "").strip()
    or os.environ.get("ES_ENDPOINT_STRATEGY", "").strip()
    or "domain"
)
if OPENSEARCH_ENDPOINT_STRATEGY == "off":
    OPENSEARCH_ENDPOINT_STRATEGY = "port"
OPENSEARCH_MULTI_CLUSTER = is_env_not_false("OPENSEARCH_MULTI_CLUSTER") or is_env_true(
    "ES_MULTI_CLUSTER"
)
LEGACY_SNS_GCM_PUBLISHING = is_env_true("LEGACY_SNS_GCM_PUBLISHING")
MAIN_DOCKER_NETWORK = os.environ.get("MAIN_DOCKER_NETWORK", "") or LAMBDA_DOCKER_NETWORK
PARITY_AWS_ACCESS_KEY_ID = is_env_true("PARITY_AWS_ACCESS_KEY_ID")
CFN_VERBOSE_ERRORS = is_env_true("CFN_VERBOSE_ERRORS")
CFN_IGNORE_UNSUPPORTED_RESOURCE_TYPES = is_env_not_false("CFN_IGNORE_UNSUPPORTED_RESOURCE_TYPES")
CFN_RESOURCE_PROVIDER_OVERRIDES = os.environ.get("CFN_RESOURCE_PROVIDER_OVERRIDES", "{}")
CONFIG_ENV_VARS = [
    "ALLOW_NONSTANDARD_REGIONS",
    "BUCKET_MARKER_LOCAL",
    "CFN_IGNORE_UNSUPPORTED_RESOURCE_TYPES",
    "CFN_VERBOSE_ERRORS",
    "CFN_RESOURCE_PROVIDER_OVERRIDES",