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.


environment['AWS_LAMBDA_FUNCTION_NAME'] = context.function_name
            environment['AWS_LAMBDA_FUNCTION_VERSION'] = context.function_version
            environment['AWS_LAMBDA_FUNCTION_INVOKED_ARN'] = context.invoked_function_arn
        
        command = ''
        
        if is_java_lambda(runtime):
            java_opts = Util.get_java_opts()
            stdin = None
            
            target_file = os.path.join(lambda_cwd, os.path.basename(LAMBDA_EXECUTOR_JAR))
            if not os.path.exists(target_file):
                cp_r(LAMBDA_EXECUTOR_JAR, target_file)
            
            taskdir = '/var/task'
            save_file(os.path.join(lambda_cwd, LAMBDA_EVENT_FILE), event_body)
            classpath = Util.get_java_classpath(target_file)
            command = ("bash -c 'cd %s; java %s -cp \"%s\" \"%s\" \"%s\" \"%s\"'" %
                (taskdir, java_opts, classpath, LAMBDA_EXECUTOR_CLASS, handler, LAMBDA_EVENT_FILE))
        
        cmd = self.prepare_execution(func_arn, environment, runtime, command, handler, lambda_cwd)
        
        LOG.info('Running lambda cmd: %s' % cmd)
        result, log_output = self.run_lambda_executor(cmd, stdin, environment)
        log_formatted = log_output.strip().replace('\n', '\n> ')
        LOG.debug('Lambda %s result / log output:\n%s\n>%s' % (func_arn, result.strip(), log_formatted))
        return result, log_output
class LambdaExecutorReuseContainers(LambdaExecutorContainers):
    """ Executor class for executing Lambda functions in re-usable Docker containers """
    def __init__(self):