### 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_
LAMBDA_JAVA_OPTS = os.environ.get("LAMBDA_JAVA_OPTS", "").strip()
KINESIS_SHARD_LIMIT = os.environ.get("KINESIS_SHARD_LIMIT", "").strip() or "100"
KINESIS_LATENCY = os.environ.get("KINESIS_LATENCY", "").strip() or "500"
KINESIS_MOCK_PERSIST_INTERVAL = os.environ.get("KINESIS_MOCK_PERSIST_INTERVAL", "").strip() or "5s"
KINESIS_PROVIDER = os.environ.get("KINESIS_PROVIDER") or "kinesis-mock"
SYNCHRONOUS_SNS_EVENTS = is_env_true("SYNCHRONOUS_SNS_EVENTS")
SYNCHRONOUS_SQS_EVENTS = is_env_true("SYNCHRONOUS_SQS_EVENTS")
SYNCHRONOUS_API_GATEWAY_EVENTS = is_env_not_false("SYNCHRONOUS_API_GATEWAY_EVENTS")
SYNCHRONOUS_KINESIS_EVENTS = is_env_not_false("SYNCHRONOUS_KINESIS_EVENTS")
SYNCHRONOUS_DYNAMODB_EVENTS = is_env_not_false("SYNCHRONOUS_DYNAMODB_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"
HOSTNAME_FROM_LAMBDA = os.environ.get("HOSTNAME_FROM_LAMBDA", "").strip()
LAMBDA_REMOTE_DOCKER = is_env_true("LAMBDA_REMOTE_DOCKER")
if is_in_docker and not os.environ.get("LAMBDA_REMOTE_DOCKER", "").strip():
    LAMBDA_REMOTE_DOCKER = True
BUCKET_MARKER_LOCAL = (
    os.environ.get("BUCKET_MARKER_LOCAL", "").strip() or DEFAULT_BUCKET_MARKER_LOCAL