### 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_
global DATA_DIR, PERSISTENCE
    if DATA_DIR:
        
        
        os.environ["PERSISTENCE"] = "1"
        PERSISTENCE = True
    if is_in_docker:
        dirs = Directories.for_container()
    else:
        dirs = Directories.for_host()
    if PERSISTENCE:
        if DATA_DIR:
            LOG.warning(
                "Persistence mode was activated using the DATA_DIR variable. DATA_DIR is deprecated and "
                "its value is ignored. The data is instead stored into %s in the LocalStack volume.",
                dirs.data,
            )
        
        DATA_DIR = dirs.data
        os.environ["DATA_DIR"] = dirs.data  
    return dirs
dirs: Directories
if LEGACY_DIRECTORIES:
    CLEAR_TMP_FOLDER = False
    dirs = init_legacy_directories()
else:
    dirs = init_directories()