When I attempt to upload a function to my Localstack instance where the runtime is specified as `provided` aka custom runtime

https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html


> To use a custom runtime, set your function's runtime to provided.
> ...
> If there's a file named `bootstrap` in your deployment package, Lambda executes that file. If not, Lambda looks for a runtime in the function's layers. If the bootstrap file isn't found or isn't executable, your function returns an error upon invocation.


So here's what's happening, when I upload the function to Localstack I get a Boto error:

```
File "/opt/code/localstack/localstack/services/cloudformation/cloudformation_starter.py", line 173, in parse_and_create_resource
localstack_1  |     return _parse_and_create_resource(logical_id, resource_json, resources_map, region_name)
localstack_1  |   File "/opt/code/localstack/localstack/services/cloudformation/cloudformation_starter.py", line 260, in _parse_and_create_resource
localstack_1  |     result = deploy_func(logical_id, resource_wrapped, stack_name=stack_name)
localstack_1  |   File "/opt/code/localstack/localstack/utils/cloudformation/template_deployer.py", line 715, in deploy_resource
localstack_1  |     result = deploy_resource_via_sdk_function(resource_id, resources, resource_type, func, stack_name)
localstack_1  |   File "/opt/code/localstack/localstack/utils/cloudformation/template_deployer.py", line 785, in deploy_resource_via_sdk_function
localstack_1  |     raise e
localstack_1  |   File "/opt/code/localstack/localstack/utils/cloudformation/template_deployer.py", line 782, in deploy_resource_via_sdk_function
localstack_1  |     result = function(**params)
localstack_1  |   File "/opt/code/localstack/.venv/lib/python3.7/site-packages/botocore/client.py", line 272, in _api_call
localstack_1  |     return self._make_api_call(operation_name, kwargs)
localstack_1  |   File "/opt/code/localstack/.venv/lib/python3.7/site-packages/botocore/client.py", line 576, in _make_api_call
localstack_1  |     raise error_class(parsed_response, operation_name)
localstack_1  | botocore.exceptions.ClientError: An error occurred (ValidationError) when calling the CreateFunction operation: Unable to find handler script 
```

The last item in this stack trace is the issue:

> botocore.exceptions.ClientError: An error occurred (ValidationError) when calling the CreateFunction operation: Unable to find handler script 

So it looks like it searching for `myHandler.sh` where it's appending that `.sh` or something is and that file doesn't exist so it's dying.

Per the amazon docs it should just be looking for a file name `bootstrap` if the lambda runtime is set to `provided`.

Now it's entirely possible this is an issue in Boto, but jumping to line **576** per the stack trace in the Boto source shows it's just emitting the http error, which leads me to think it's in Localstack .

**... Later ...**

Ok upon further inspection it looks like this if the offending line:

https://github.com/localstack/localstack/blob/master/localstack/services/awslambda/lambda_api.py#L420-L421

It's explicitly affixing that `.sh` extension. May AWS changed things around around how they handle "bootstrap" and at some point it in the past it was "bootstrap.sh" I dunno.. Regardless, I'm thinking the solve, which I will submit if agreed upon, is that for a `runtime` type of `provided` it should be searching for `bootstrap`  no extension at all.  I don't even know that it needs to set the handler at all.. Maybe it does for Localstack.


if not isinstance(lambda_api.LAMBDA_EXECUTOR,
                          lambda_executors.LambdaExecutorSeparateContainers):
            return
        executor = lambda_api.LAMBDA_EXECUTOR
        func_name = 'test_docker_command_for_separate_container_lambda_executor'
        func_arn = lambda_api.func_arn(func_name)
        handler = 'handler'
        lambda_cwd = '/app/lambda'
        network = 'compose_network'
        config.LAMBDA_DOCKER_NETWORK = network
        cmd = executor.prepare_execution(func_arn, {}, LAMBDA_RUNTIME_NODEJS810, '', handler, lambda_cwd)
        expected = 'docker run -v "%s":/var/task   --network="%s"  --rm "lambci/lambda:%s" "%s"' % (
            lambda_cwd, network, LAMBDA_RUNTIME_NODEJS810, handler)
        self.assertIn(('--network="%s"' % network), cmd, 'cmd=%s expected=%s' % (cmd, expected))
        config.LAMBDA_DOCKER_NETWORK = ''
    def test_destroy_idle_containers(self):
        
        if not isinstance(lambda_api.LAMBDA_EXECUTOR,
                          lambda_executors.LambdaExecutorReuseContainers):
            return
        executor = lambda_api.LAMBDA_EXECUTOR
        func_name = 'test_destroy_idle_containers'
        func_arn = lambda_api.func_arn(func_name)
        
        executor.destroy_existing_docker_containers()
        self.assertEqual(len(executor.get_all_container_names()), 0)
        
        zip_file = testutil.create_lambda_archive(
            load_file(TEST_LAMBDA_ENV),
            get_content=True,