Manager crashes on startup

```
connected to master
Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/urllib3/connectionpool.py", line 670, in urlopen
httplib_response = self._make_request(
File "/usr/local/lib/python3.8/site-packages/urllib3/connectionpool.py", line 392, in _make_request
conn.request(method, url, **httplib_request_kw)
File "/usr/local/lib/python3.8/http/client.py", line 1230, in request
self._send_request(method, url, body, headers, encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1276, in _send_request
self.endheaders(body, encode_chunked=encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1225, in endheaders
self._send_output(message_body, encode_chunked=encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1004, in _send_output
self.send(msg)
File "/usr/local/lib/python3.8/http/client.py", line 944, in send
self.connect()
File "/usr/local/lib/python3.8/site-packages/docker/transport/unixconn.py", line 43, in connect
sock.connect(self.unix_socket)
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/requests/adapters.py", line 439, in send
resp = conn.urlopen(
File "/usr/local/lib/python3.8/site-packages/urllib3/connectionpool.py", line 724, in urlopen
retries = retries.increment(
File "/usr/local/lib/python3.8/site-packages/urllib3/util/retry.py", line 403, in increment
raise six.reraise(type(error), error, _stacktrace)
File "/usr/local/lib/python3.8/site-packages/urllib3/packages/six.py", line 734, in reraise
raise value.with_traceback(tb)
File "/usr/local/lib/python3.8/site-packages/urllib3/connectionpool.py", line 670, in urlopen
httplib_response = self._make_request(
File "/usr/local/lib/python3.8/site-packages/urllib3/connectionpool.py", line 392, in _make_request
conn.request(method, url, **httplib_request_kw)
File "/usr/local/lib/python3.8/http/client.py", line 1230, in request
self._send_request(method, url, body, headers, encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1276, in _send_request
self.endheaders(body, encode_chunked=encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1225, in endheaders
self._send_output(message_body, encode_chunked=encode_chunked)
File "/usr/local/lib/python3.8/http/client.py", line 1004, in _send_output
self.send(msg)
File "/usr/local/lib/python3.8/http/client.py", line 944, in send
self.connect()
File "/usr/local/lib/python3.8/site-packages/docker/transport/unixconn.py", line 43, in connect
sock.connect(self.unix_socket)
urllib3.exceptions.ProtocolError: ('Connection aborted.', ConnectionRefusedError(111, 'Connection refused'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "./manager.py", line 114, in
main()
File "./manager.py", line 110, in main
docker_checker()
File "./manager.py", line 78, in docker_checker
this_container = client.containers.get(my_hostname)
File "/usr/local/lib/python3.8/site-packages/docker/models/containers.py", line 880, in get
resp = self.client.api.inspect_container(container_id)
File "/usr/local/lib/python3.8/site-packages/docker/utils/decorators.py", line 19, in wrapped
return f(self, resource_id, *args, **kwargs)
File "/usr/local/lib/python3.8/site-packages/docker/api/container.py", line 758, in inspect_container
self._get(self._url("/containers/{0}/json", container)), True
File "/usr/local/lib/python3.8/site-packages/docker/utils/decorators.py", line 46, in inner
return f(self, *args, **kwargs)
File "/usr/local/lib/python3.8/site-packages/docker/api/client.py", line 230, in _get
return self.get(url, **self._set_request_timeout(kwargs))
File "/usr/local/lib/python3.8/site-packages/requests/sessions.py", line 543, in get
return self.request('GET', url, **kwargs)
File "/usr/local/lib/python3.8/site-packages/requests/sessions.py", line 530, in request
resp = self.send(prep, **send_kwargs)
File "/usr/local/lib/python3.8/site-packages/requests/sessions.py", line 643, in send
r = adapter.send(request, **kwargs)
File "/usr/local/lib/python3.8/site-packages/requests/adapters.py", line 498, in send
raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', ConnectionRefusedError(111, 'Connection refused'))
citus_manager exited with code 1
```

[Docker rootless](https://docs.docker.com/engine/security/rootless/) on Ubuntu 20.04
Docker version 19.03.11, build 42e35e61f3
docker-compose version 1.26.0, build unknown

Start with manager.py, especially main() and docker_checker() around the reported line 78, then reproduce the startup failure using Docker rootless on Ubuntu 20.04 with the listed Docker and docker-compose versions. Trace how the Docker client is configured and define completion as the manager starting without the reported ConnectionRefusedError in that environment.

Written by the indexing model from the issue text.