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.
folder = new_tmp_file()
    rm_rf(folder)
    mkdir(folder)
    return folder
def is_ip_address(addr):
    try:
        socket.inet_aton(addr)
        return True
    except socket.error:
        return False
def is_zip_file(content):
    stream = BytesIO(content)
    return zipfile.is_zipfile(stream)
def unzip(path, target_dir):
    try:
        zip_ref = zipfile.ZipFile(path, 'r')
    except Exception as e:
        LOG.warning('Unable to open zip file: %s: %s' % (path, e))
        raise e
    
    
    for file_entry in zip_ref.infolist():
        _unzip_file_entry(zip_ref, file_entry, target_dir)
    zip_ref.close()
def _unzip_file_entry(zip_ref, file_entry, target_dir):