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.
if files:
        cleanup_tmp_files()
def cleanup_threads_and_processes(quiet=True):
    for t in TMP_THREADS:
        t.stop(quiet=quiet)
    for p in TMP_PROCESSES:
        try:
            p.terminate()
        except Exception as e:
            print(e)
    
    clear_list(TMP_THREADS)
    clear_list(TMP_PROCESSES)
def clear_list(l):
    while len(l):
        del l[0]
def cleanup_tmp_files():
    for tmp in TMP_FILES:
        try:
            rm_rf(tmp)
        except Exception:
            pass  
    del TMP_FILES[:]
def new_tmp_file():
    """ Return a path to a new temporary file. """
    tmp_file, tmp_path = tempfile.mkstemp()
    os.close(tmp_file)
    TMP_FILES.append(tmp_path)
    return tmp_path
def new_tmp_dir():