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.
pass
def make_http_request(url, data=None, headers=None, method='GET'):
    if is_string(method):
        method = requests.__dict__[method.lower()]
    return method(url, headers=headers, data=data, auth=NetrcBypassAuth(), verify=False)
class SafeStringIO(io.StringIO):
    """ Safe StringIO implementation that doesn't fail if str is passed in Python 2. """
    def write(self, obj):
        if six.PY2 and isinstance(obj, str):
            obj = obj.decode('unicode-escape')
        return super(SafeStringIO, self).write(obj)
def clean_cache(file_pattern=CACHE_FILE_PATTERN,
        last_clean_time=last_cache_clean_time, max_age=CACHE_MAX_AGE):
    mutex_clean.acquire()
    time_now = now()
    try:
        if last_clean_time['time'] > time_now - CACHE_CLEAN_TIMEOUT:
            return
        for cache_file in set(glob.glob(file_pattern)):
            mod_time = os.path.getmtime(cache_file)
            if time_now > mod_time + max_age:
                rm_rf(cache_file)
        last_clean_time['time'] = time_now
    finally:
        mutex_clean.release()
    return time_now
def truncate(data, max_length=100):