### 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_
):
            return False
    except Exception:
        pass
    with open("/proc/1/cgroup", "rt") as ifh:
        content = ifh.read()
        if "docker" in content:
            return True
        os_hostname = socket.gethostname()
        if os_hostname and os_hostname in content:
            return True
    return False
OVERRIDE_IN_DOCKER = is_env_true("OVERRIDE_IN_DOCKER")
is_in_docker = in_docker()
is_in_linux = is_linux()
CONFIG_PROFILE = os.environ.get("CONFIG_PROFILE", "").strip()
CONFIG_DIR = os.environ.get("CONFIG_DIR", os.path.expanduser("~/.localstack"))
try:
    load_environment(CONFIG_PROFILE)
except ImportError:
    
    pass
if "DEFAULT_REGION" not in os.environ:
    os.environ["DEFAULT_REGION"] = os.environ.get("AWS_DEFAULT_REGION") or AWS_REGION_US_EAST_1
DEFAULT_REGION = os.environ["DEFAULT_REGION"]
HOSTNAME_EXTERNAL = os.environ.get("HOSTNAME_EXTERNAL", "").strip() or LOCALHOST
LOCALSTACK_HOSTNAME = os.environ.get("LOCALSTACK_HOSTNAME", "").strip() or LOCALHOST
DATA_DIR = os.environ.get("DATA_DIR", "").strip()
PERSISTENCE = is_env_true("PERSISTENCE")
CLEAR_TMP_FOLDER = is_env_not_false("CLEAR_TMP_FOLDER")