I'm trying to start localstack in a containerized Jenkins build agent for unit testing. In other words, trying to start the localstack docker container from within a docker container. We do this all the time with other images, using the method of mounting `/var/run/docker.sock` in the "parent" container, but localstack will not start.

I get this error:

```
2019-11-08T18:53:53:ERROR:localstack.services.generic_proxy: Exception running proxy on port 8081: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem' Traceback (most recent call last):
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 384, in run_cmd
    combined_file, cert_file_name, key_file_name = GenericProxy.create_ssl_cert(serial_number=self.port)
  File "/opt/code/localstack/localstack/services/generic_proxy.py", line 401, in create_ssl_cert
    return generate_ssl_cert(SERVER_CERT_PEM_FILE, serial_number=serial_number)
  File "/opt/code/localstack/localstack/utils/common.py", line 828, in generate_ssl_cert
    save_file(target_file, file_content)
  File "/opt/code/localstack/localstack/utils/common.py", line 642, in save_file
    with open(file, mode) as f:
PermissionError: [Errno 13] Permission denied: '/tmp/localstack/server.test.pem'
```

Port 8081 is not already in use, and `/tmp/localstack` is writable, so I don't understand why it's getting permission denied.
#set( $maxIndex = $numRecords - 1 )
            #foreach( $idx in [0..$maxIndex] )
            #set( $elem = $input.path("$.records[${idx}]") )
            #set( $elemJsonB64 = $util.base64Encode($elem.data) )
            {
                "Data": "$elemJsonB64",
                "PartitionKey": #if( $elem.partitionKey != '')"$elem.partitionKey"
                                #else"$elemJsonB64.length()"#end
            }#if($foreach.hasNext),#end
            #end
            #end
        ]
    }"""
    
    API_PATH_DATA_INBOUND = '/data'
    API_PATH_HTTP_BACKEND = '/hello_world'
    API_PATH_LAMBDA_PROXY_BACKEND = '/lambda/foo1'
    API_PATH_LAMBDA_PROXY_BACKEND_WITH_PATH_PARAM = '/lambda/{test_param1}'
    API_PATH_LAMBDA_PROXY_BACKEND_ANY_METHOD = '/lambda-any-method/foo1'
    API_PATH_LAMBDA_PROXY_BACKEND_ANY_METHOD_WITH_PATH_PARAM = '/lambda-any-method/{test_param1}'
    
    TEST_STREAM_KINESIS_API_GW = 'test-stream-api-gw'
    TEST_SQS_QUEUE = 'test-sqs-queue-api-gw'
    TEST_STAGE_NAME = 'testing'
    TEST_LAMBDA_PROXY_BACKEND = 'test_lambda_apigw_backend'
    TEST_LAMBDA_PROXY_BACKEND_WITH_PATH_PARAM = 'test_lambda_apigw_backend_path_param'
    TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD = 'test_ARMlambda_apigw_backend_any_method'
    TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD_WITH_PATH_PARAM = 'test_ARMlambda_apigw_backend_any_method_path_param'
    TEST_LAMBDA_HANDLER_NAME = 'lambda_sqs_handler'
    def test_api_gateway_kinesis_integration(self):