### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

There's no way to explicitly toggle the `-inMemory` flag for DynamoDB - it relies on [`db_path` being unset](https://github.com/localstack/localstack/blob/v1.0.1/localstack/services/dynamodb/server.py#L45). However, `db_path` is always set because it [relies on config.dirs.data](https://github.com/localstack/localstack/blob/v1.0.1/localstack/services/dynamodb/server.py#L107-L113), which is [always set](https://github.com/localstack/localstack/blob/v1.0.1/localstack/config.py#L119).

### Expected Behavior

Maybe have an explicit env var like `DYNAMODB_IN_MEMORY` to force `-inMemory` mode or fix the path based logic in which it can be enabled.

Use case: I'm running tests on an ephemeral localstack container and don't need persistence. I think I'm experiencing the performance hit in https://github.com/localstack/localstack/issues/1205 and `-inMemory` seems be significantly faster.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

`docker-compose.yml`
```yaml
version: '3.8'
services:
  localstack:
    image: localstack/localstack:1.0
    environment:
      - EDGE_PORT=4566
      - EAGER_SERVICE_LOADING=1
      - SERVICES=dynamodb
      - PERSISTENCE=0
```

```
$ docker-compose up
...
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : Initializing DynamoDB Local with the following configuration:
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : Port:	34585
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : InMemory:	false
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : DbPath:	/var/lib/localstack/tmp/state/dynamodb
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : SharedDb:	false
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : shouldDelayTransientStatuses:	false
localstack_1      | 2022-07-23T05:27:50.506  INFO --- [   Thread-12] l.services.dynamodb.server : CorsParams:	*
...
```

### Environment

```markdown
- OS: Docker on Mac M1
- LocalStack: 1.0.1
```


### Anything else?

_No response_
if LS_LOG:
        log_level = str(LS_LOG).upper()
        return log_level.lower() in TRACE_LOG_LEVELS
    return False
if DEBUG:
    logging.getLogger("").setLevel(logging.DEBUG)
    logging.getLogger("localstack").setLevel(logging.DEBUG)
LOG = logging.getLogger(__name__)
if is_trace_logging_enabled():
    load_end_time = time.time()
    LOG.debug(
        "Initializing the configuration took %s ms", int((load_end_time - load_start_time) * 1000)
    )
def parse_service_ports() -> Dict[str, int]:
    """Parses the environment variable $SERVICES with a comma-separated list of services
    and (optional) ports they should run on: 'service1:port1,service2,service3:port3'"""
    service_ports = os.environ.get("SERVICES", "").strip()
    if service_ports and not is_env_true("EAGER_SERVICE_LOADING"):
        LOG.warning("SERVICES variable is ignored if EAGER_SERVICE_LOADING=0.")
        service_ports = None  
    if not service_ports:
        return DEFAULT_SERVICE_PORTS
    result = {}
    for service_port in re.split(r"\s*,\s*", service_ports):
        parts = re.split(r"[:=]", service_port)
        service = parts[0]
        key_upper = service.upper().replace("-", "_")
        port_env_name = "%s_PORT" % key_upper
        
        port_number = DEFAULT_SERVICE_PORTS.get(service)