### 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 self.db_path is None
    @property
    def jar_path(self) -> str:
        return f"{dirs.static_libs}/dynamodb/DynamoDBLocal.jar"
    @property
    def library_path(self) -> str:
        return f"{dirs.static_libs}/dynamodb/DynamoDBLocal_lib"
    def _create_shell_command(self) -> List[str]:
        cmd = [
            "java",
            "-Xmx%s" % self.heap_size,
            f"-javaagent:{DDB_AGENT_JAR_PATH}",
            f"-Djava.library.path={self.library_path}",
            "-jar",
            self.jar_path,
        ]
        parameters = []
        parameters.extend(["-port", str(self.port)])
        if self.in_memory:
            parameters.append("-inMemory")
        if self.db_path:
            parameters.extend(["-dbPath", self.db_path])
        if self.delay_transient_statuses:
            parameters.extend(["-delayTransientStatuses"])
        if self.optimize_db_before_startup:
            parameters.extend(["-optimizeDbBeforeStartup"])
        if self.share_db:
            parameters.extend(["-sharedDb"])
        return cmd + parameters
    def do_start_thread(self) -> FuncThread: