### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

First of all, I'd like to thank you for this awesome project. Keep up the good work.

Now, to business. I have a script that tries to set the executor at runtime (from `docker-reuse` to `docker` and back again at the end) for a very specific use case. In my `docker-compose` file, I have set the `ENABLE_CONFIG_UPDATES=1` and upon POSTing the request to `/?_config_`, I see the following log message in the logs:

```shell
localstack_1  | 2021-07-26T07:03:35:INFO:localstack.services.infra: Updating value of config variable "LAMBDA_EXECUTOR": docker
````
That said, I was expecting that the next invocation of a Lambda would be using its own container, which it doesn't:

```
localstack.services.awslambda.lambda_executors: Command for docker-reuse Lambda executor:
```

### Expected Behavior

The next invocation of a Lambda would be in its own container.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

1. Create a simple `docker-compose.yml` like so:
    ```yaml
    localstack:
      image: localstack/localstack:0.12.15
      ports:
        - "4566:4566"
      environment:
        SERVICES: lambda,cloudformation,s3,cloudwatch,iam,apigateway
        DEFAULT_REGION: eu-west-1
        HOSTNAME_EXTERNAL: localstack
        LAMBDA_EXECUTOR: docker-reuse
        LAMBDA_DOCKER_NETWORK: offline-environment_default
        LAMBDA_REMOTE_DOCKER: 0
        DOCKER_HOST: unix:///var/run/docker.sock
        DEBUG: 1
        ENABLE_CONFIG_UPDATES: 1
        HOST_TMP_FOLDER: /private${TMPDIR}/localstack
        AWS_ACCESS_KEY_ID: test
        AWS_SECRET_ACCESS_KEY: test
      volumes:
        - "./localstack/scripts:/docker-entrypoint-initaws.d"
        - "./localstack/cloudformation:/cloudformation"
        - "/var/run/docker.sock:/var/run/docker.sock"
        - "/private${TMPDIR}/localstack:/tmp/localstack"
    ```
2. Create a CloudFormation template with a Lambda and an S3 bucket (and their required permissions) and place it under the `localstack/cloudformation` directory. The Lambda should be triggered by an S3 object created event (may not matter). 
3. Create a bash script that deploys said CloudFormation template and place it under the `localstack/scripts` directory with the prefix `1_`.
4. Create another bash script that changes the value of `LAMBDA_EXECUTOR` to `docker` (via POSTing to `/?_config_`) and copies a file into the bucket. Again, place the script under the `localstack/scripts` directory with the prefix `2_`.
5. Start the `docker-compose.yml`:
    ```shell
    docker-compose-up
    ```
6. Inspect the logs and verify that everything was deployed correctly and that you see the message `Updating value of config variable "LAMBDA_EXECUTOR": docker`
7. Open the Docker Dashboard or issue the `docker ps` command to confirm that Localstack invoked the Lambda using the `docker-reuse` executor. Can also check the logs and verify that the following message appears:
    ```shell
    localstack.services.awslambda.lambda_executors: Command for docker-reuse Lambda executor:
    ```


### Environment

```markdown
- OS: macOS Big Sur 11.5
- LocalStack: 0.12.15
```
import base64
import functools
import hashlib
import importlib.machinery
import json
import logging
import os
import re
import sys
import threading
import time
import traceback
import uuid
from datetime import datetime
from io import BytesIO
from flask import Flask, Response, jsonify, request
from six.moves import cStringIO as StringIO
from six.moves.urllib.parse import urlparse
from localstack import config
from localstack.constants import APPLICATION_JSON, TEST_AWS_ACCOUNT_ID
from localstack.services.awslambda import lambda_executors
from localstack.services.awslambda.lambda_utils import (
    DOTNET_LAMBDA_RUNTIMES,