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.


docker_cmd = self._docker_cmd()
            status = self.get_docker_container_status(func_arn)
            LOG.debug('Priming docker container (status "%s"): %s' % (status, container_name))
            docker_image = Util.docker_image_for_runtime(runtime)
            rm_flag = Util.get_docker_remove_flag()
            
            if status < 1:
                
                self.destroy_docker_container(func_arn)
                env_vars_str = ' '.join(['-e {}={}'.format(k, cmd_quote(v)) for (k, v) in env_vars])
                network = config.LAMBDA_DOCKER_NETWORK
                network_str = '--network="%s"' % network if network else ''
                mount_volume = not config.LAMBDA_REMOTE_DOCKER
                lambda_cwd_on_host = Util.get_host_path_for_path_in_docker(lambda_cwd)
                mount_volume_str = '-v "%s":/var/task' % lambda_cwd_on_host if mount_volume else ''
                
                LOG.debug('Creating container: %s' % container_name)
                cmd = (
                    '%s create'
                    ' %s'  
                    ' --name "%s"'
                    ' --entrypoint /bin/bash'  
                    ' %s'
                    ' --interactive'  
                    ' -e AWS_LAMBDA_EVENT_BODY="$AWS_LAMBDA_EVENT_BODY"'
                    ' -e HOSTNAME="$HOSTNAME"'
                    ' -e LOCALSTACK_HOSTNAME="$LOCALSTACK_HOSTNAME"'
                    '  %s'  
                    '  %s'  
                    ' %s'