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.


LAMBDA_DEFAULT_HANDLER = 'handler.handler'
LAMBDA_DEFAULT_RUNTIME = LAMBDA_RUNTIME_PYTHON27
LAMBDA_DEFAULT_STARTING_POSITION = 'LATEST'
LAMBDA_ZIP_FILE_NAME = 'original_lambda_archive.zip'
LAMBDA_JAR_FILE_NAME = 'original_lambda_archive.jar'
app = Flask(APP_NAME)
arn_to_lambda = {}
event_source_mappings = []
LOG = logging.getLogger(__name__)
exec_mutex = threading.Semaphore(1)
DO_USE_DOCKER = None
JSON_START_CHAR_MAP = {
    list: ('[',),
    tuple: ('[',),
    dict: ('{',),
    str: ('"',),
    bytes: ('"',),
    bool: ('t', 'f'),
    type(None): ('n',),
    int: ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9'),
    float: ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9')
}
POSSIBLE_JSON_TYPES = (str, bytes)
JSON_START_TYPES = tuple(set(JSON_START_CHAR_MAP.keys()) - set(POSSIBLE_JSON_TYPES))
JSON_START_CHARS = tuple(set(functools.reduce(lambda x, y: x + y, JSON_START_CHAR_MAP.values())))
LAMBDA_EXECUTOR = lambda_executors.AVAILABLE_EXECUTORS.get(config.LAMBDA_EXECUTOR, lambda_executors.DEFAULT_EXECUTOR)
IAM_POLICY_VERSION = '2012-10-17'
POLICY_NAME_PATTERN = 'lambda_policy_%s'
BUCKET_MARKER_LOCAL = '__local__'
class ClientError(Exception):