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.
"""Downloads file at url to the given path"""
    
    
    s = requests.Session()
    r = s.get(url, stream=True, verify=verify_ssl)
    total = 0
    try:
        if not os.path.exists(os.path.dirname(path)):
            os.makedirs(os.path.dirname(path))
        LOG.debug('Starting download from %s to %s (%s bytes)' % (url, path, r.headers.get('content-length')))
        with open(path, 'wb') as f:
            for chunk in r.iter_content(DOWNLOAD_CHUNK_SIZE):
                total += len(chunk)
                if chunk:  
                    f.write(chunk)
                    LOG.debug('Writing %s bytes (total %s) to %s' % (len(chunk), total, path))
                else:
                    LOG.debug('Empty chunk %s (total %s) from %s' % (chunk, total, url))
            f.flush()
            os.fsync(f)
    finally:
        LOG.debug('Done downloading %s, response code %s' % (url, r.status_code))
        r.close()
        s.close()
def parse_chunked_data(data):