detect missing docker permissions and provide a useful prompt

[tfoote]

Right now you get a long traceback if you don't have permissions to build the docker container

$ rocker docker/hello-world                                                    
Plugins found: ['dev_helpers', 'home', 'nvidia', 'pulse', 'user']
Active extensions []                                                           
Writing dockerfile to /tmp/tmp93bgqbui/Dockerfile
vvvvvv                                                                                                

FROM docker/hello-world

^^^^^^
Building docker file with arguments:  {'path': '/tmp/tmp93bgqbui', 'rm': True, 'decode': True, 'nocache': False, 'tag': 'rocker_docker/hello-world'}
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 601, in urlopen
    chunked=chunked)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 357, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/usr/lib/python3.6/http/client.py", line 1239, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1285, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1234, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1026, in _send_output
    self.send(msg)
  File "/usr/lib/python3.6/http/client.py", line 964, in send
    self.connect()
  File "/usr/lib/python3/dist-packages/docker/transport/unixconn.py", line 33, in connect
    sock.connect(self.unix_socket)
PermissionError: [Errno 13] Permission denied

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 440, in send
    timeout=timeout
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 639, in urlopen
    _stacktrace=sys.exc_info()[2])
  File "/usr/lib/python3/dist-packages/urllib3/util/retry.py", line 357, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File "/usr/lib/python3/dist-packages/six.py", line 692, in reraise
    raise value.with_traceback(tb)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 601, in urlopen
    chunked=chunked)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 357, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/usr/lib/python3.6/http/client.py", line 1239, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1285, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1234, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/lib/python3.6/http/client.py", line 1026, in _send_output
    self.send(msg)
  File "/usr/lib/python3.6/http/client.py", line 964, in send
    self.connect()
  File "/usr/lib/python3/dist-packages/docker/transport/unixconn.py", line 33, in connect
    sock.connect(self.unix_socket)
urllib3.exceptions.ProtocolError: ('Connection aborted.', PermissionError(13, 'Permission denied'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/rocker", line 11, in <module>
    load_entry_point('rocker==0.1.3', 'console_scripts', 'rocker')()
  File "/usr/lib/python3/dist-packages/rocker/cli.py", line 63, in main
    exit_code = dig.build(**vars(args))
  File "/usr/lib/python3/dist-packages/rocker/core.py", line 64, in build
    for line in docker_client.build(**arguments):
  File "/usr/lib/python3/dist-packages/docker/api/build.py", line 246, in build
    timeout=timeout,
  File "/usr/lib/python3/dist-packages/docker/utils/decorators.py", line 46, in inner
    return f(self, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/docker/api/client.py", line 185, in _post
    return self.post(url, **self._set_request_timeout(kwargs))
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 567, in post
    return self.request('POST', url, data=data, json=json, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 520, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 630, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 490, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', PermissionError(13, 'Permission denied'))