### 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_
os.environ.get("EXTERNAL_SERVICE_PORTS_END")
    or os.environ.get("SERVICE_INSTANCES_PORTS_END")
    or (EXTERNAL_SERVICE_PORTS_START + 50)
)
LAMBDA_JAVA_OPTS = os.environ.get("LAMBDA_JAVA_OPTS", "").strip()
KINESIS_SHARD_LIMIT = os.environ.get("KINESIS_SHARD_LIMIT", "").strip() or "100"
KINESIS_ON_DEMAND_STREAM_COUNT_LIMIT = (
    os.environ.get("KINESIS_ON_DEMAND_STREAM_COUNT_LIMIT", "").strip() or "10"
)
KINESIS_LATENCY = os.environ.get("KINESIS_LATENCY", "").strip() or "500"
KINESIS_MOCK_PERSIST_INTERVAL = os.environ.get("KINESIS_MOCK_PERSIST_INTERVAL", "").strip() or "5s"
SYNCHRONOUS_KINESIS_EVENTS = is_env_not_false("SYNCHRONOUS_KINESIS_EVENTS")  
KINESIS_ERROR_PROBABILITY = float(os.environ.get("KINESIS_ERROR_PROBABILITY", "").strip() or 0.0)
DYNAMODB_ERROR_PROBABILITY = float(os.environ.get("DYNAMODB_ERROR_PROBABILITY", "").strip() or 0.0)
DYNAMODB_READ_ERROR_PROBABILITY = float(
    os.environ.get("DYNAMODB_READ_ERROR_PROBABILITY", "").strip() or 0.0
)
DYNAMODB_WRITE_ERROR_PROBABILITY = float(
    os.environ.get("DYNAMODB_WRITE_ERROR_PROBABILITY", "").strip() or 0.0
)
DYNAMODB_HEAP_SIZE = os.environ.get("DYNAMODB_HEAP_SIZE", "").strip() or "256m"
DYNAMODB_SHARE_DB = int(os.environ.get("DYNAMODB_SHARE_DB") or 0)
SQS_DELAY_PURGE_RETRY = is_env_true("SQS_DELAY_PURGE_RETRY")
SQS_DELAY_RECENTLY_DELETED = is_env_true("SQS_DELAY_RECENTLY_DELETED")
SQS_PORT_EXTERNAL = int(os.environ.get("SQS_PORT_EXTERNAL") or 0)
SQS_ENDPOINT_STRATEGY = os.environ.get("SQS_ENDPOINT_STRATEGY", "") or "off"
SQS_DISABLE_MAX_NUMBER_OF_MESSAGE_LIMIT = is_env_true("SQS_DISABLE_MAX_NUMBER_OF_MESSAGE_LIMIT")
SQS_DISABLE_CLOUDWATCH_METRICS = is_env_true("SQS_DISABLE_CLOUDWATCH_METRICS")
SQS_CLOUDWATCH_METRICS_REPORT_INTERVAL = int(
    os.environ.get("SQS_CLOUDWATCH_METRICS_REPORT_INTERVAL") or 60