### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

* Create any table with any records
* Do a TransactGetItems on the table
* Localstack returns 
```
ResourceNotFoundException: Cannot do operations on a non-existent table
```

### Expected Behavior

TransactGetItems should succeed.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

```
docker-compose up
```

`docker-compose.yml`
```
version: '3'

services:
  localstack:
    image: localstack/localstack
    environment:
      - DEFAULT_REGION=eu-west-2
      - SERVICES=dynamodb
      - DOCKER_HOST=unix:///var/run/docker.sock
    ports:
      - 4566:4566
```

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

```
#! /bin/bash
aws --endpoint-url=http://localhost:4566 dynamodb create-table --attribute-definitions AttributeName=forumId,AttributeType=S --table-name forum --key-schema AttributeName=forumId,KeyType=HASH --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5 > /dev/null
aws --endpoint-url=http://localhost:4566 dynamodb put-item --table-name forum --item '{"forumId": {"S": "forum001"}, "data": {"S": "helloworld"}}' > /dev/null
aws --endpoint-url=http://localhost:4566 dynamodb get-item --table-name forum --key '{"forumId": {"S": "forum001"}}' > /dev/null
# This one fails
aws --endpoint-url=http://localhost:4566 dynamodb transact-get-items --transact-items '[{"Get": {"Key":{"forumId": {"S": "forum001"}}, "TableName": "forum"}}]' > /dev/null
```


### Environment

```markdown
- OS: MacOS 10.15.7
- LocalStack: latest, digest 843db39ff1df83fd537e15b30e39d6fb8c3590efd6bf3f6f96b1734418fbd19f
```


### Anything else?

This works in `localstack/localstack:0.14.1`
self,
        context: RequestContext,
        table_name: TableName,
        time_to_live_specification: TimeToLiveSpecification,
    ) -> UpdateTimeToLiveOutput:
        
        backend = DynamoDBRegion.get()
        backend.ttl_specifications[table_name] = time_to_live_specification
        return UpdateTimeToLiveOutput(TimeToLiveSpecification=time_to_live_specification)
    def create_global_table(
        self, context: RequestContext, global_table_name: TableName, replication_group: ReplicaList
    ) -> CreateGlobalTableOutput:
        if global_table_name in DynamoDBRegion.GLOBAL_TABLES:
            raise GlobalTableAlreadyExistsException("Global table with this name already exists")
        replication_group = [grp.copy() for grp in replication_group or []]
        data = {"GlobalTableName": global_table_name, "ReplicationGroup": replication_group}
        DynamoDBRegion.GLOBAL_TABLES[global_table_name] = data
        for group in replication_group:
            group["ReplicaStatus"] = "ACTIVE"
            group["ReplicaStatusDescription"] = "Replica active"
        return CreateGlobalTableOutput(GlobalTableDescription=data)
    def describe_global_table(