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.
self._stdout = self.LogStreamIO()
        self._stderr = self.LogStreamIO()
    def __enter__(self):
        
        from werkzeug.local import LocalProxy
        ident = self._ident()
        if ident not in self.CONTEXTS_BY_THREAD:
            self.CONTEXTS_BY_THREAD[ident] = self
            self._set(LocalProxy(self._proxy(sys.stdout, 'stdout')),
                      LocalProxy(self._proxy(sys.stderr, 'stderr')),
                      LocalProxy(self._proxy(sys.__stdout__, 'stdout')),
                      LocalProxy(self._proxy(sys.__stderr__, 'stderr')))
        return self
    def __exit__(self, type, value, traceback):
        ident = self._ident()
        removed = self.CONTEXTS_BY_THREAD.pop(ident, None)
        if not self.CONTEXTS_BY_THREAD:
            
            self._set(self.orig_stdout, self.orig_stderr, self.orig___stdout, self.orig___stderr)
        
        removed._stdout.flush()
        removed._stderr.flush()
        out = removed._stdout.getvalue()
        err = removed._stderr.getvalue()
        
        removed._stdout.close()
        removed._stderr.close()
        removed._stdout = out
        removed._stderr = err
    def _set(self, out, err, __out, __err):