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.
import shutil
import requests
import dns.resolver
from io import BytesIO
from contextlib import closing
from datetime import datetime
from six import with_metaclass
from six.moves import cStringIO as StringIO
from six.moves.urllib.parse import urlparse
from multiprocessing.dummy import Pool
from localstack import config
from localstack.config import DEFAULT_ENCODING
from localstack.constants import ENV_DEV
from localstack.utils import bootstrap
from localstack.utils.bootstrap import FuncThread
TMP_FILES = []
TMP_THREADS = []
TMP_PROCESSES = []
CACHE_CLEAN_TIMEOUT = 60 * 5
CACHE_MAX_AGE = 60 * 60
CACHE_FILE_PATTERN = os.path.join(tempfile.gettempdir(), '_random_dir_', 'cache.*.json')
last_cache_clean_time = {'time': 0}
mutex_clean = threading.Semaphore(1)
TIMESTAMP_FORMAT = '%Y-%m-%dT%H:%M:%S'
TIMESTAMP_FORMAT_MILLIS = '%Y-%m-%dT%H:%M:%S.%fZ'
CODEC_HANDLER_UNDERSCORE = 'underscore'
DOWNLOAD_CHUNK_SIZE = 1024 * 1024
LOG = logging.getLogger(__name__)
INFRA_STOPPED = False
class CustomEncoder(json.JSONEncoder):