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.
m = hashlib.md5()
    m.update(to_bytes(string))
    return m.hexdigest()
def in_docker():
    return config.in_docker()
def has_docker():
    try:
        run('docker ps')
        return True
    except Exception:
        return False
def get_docker_container_names():
    return bootstrap.get_docker_container_names()
def is_port_open(port_or_url, http_path=None, expect_success=True, protocols=['tcp']):
    port = port_or_url
    host = 'localhost'
    protocol = 'http'
    protocols = protocols if isinstance(protocols, list) else [protocols]
    if isinstance(port, six.string_types):
        url = urlparse(port_or_url)
        port = url.port
        host = url.hostname
        protocol = url.scheme
    nw_protocols = []
    nw_protocols += ([socket.SOCK_STREAM] if 'tcp' in protocols else [])
    nw_protocols += ([socket.SOCK_DGRAM] if 'udp' in protocols else [])
    for nw_protocol in nw_protocols:
        with closing(socket.socket(socket.AF_INET, nw_protocol)) as sock:
            sock.settimeout(1)
            if nw_protocol == socket.SOCK_DGRAM: