prefect: Docker Agent can't use SSH for --base-url

Description

Docker Agent can’t connect to remote Docker daemon with SSH. It results in

Traceback (most recent call last):
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\docker\api\client.py", line 165, in __init__
    self._custom_adapter = SSHHTTPAdapter(
NameError: name 'SSHHTTPAdapter' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\tfh\Miniconda3\envs\prefect\Scripts\prefect-script.py", line 9, in <module>
    sys.exit(cli())
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 782, in main
    rv = self.invoke(ctx)
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\click\decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\prefect\cli\agent.py", line 260, in start
    docker_interface=not no_docker_interface,
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\prefect\agent\docker\agent.py", line 117, in __init__
    self.docker_client = self._get_docker_client()
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\prefect\agent\docker\agent.py", line 142, in _get_docker_client
    return docker.APIClient(base_url=self.base_url, version="auto")
  File "C:\Users\tfh\Miniconda3\envs\prefect\lib\site-packages\docker\api\client.py", line 170, in __init__
    'Install paramiko package to enable ssh:// support'
docker.errors.DockerException: Install paramiko package to enable ssh:// support

Expected Behavior

That I could connect to docker host.

Reproduction

Simply run: prefect agent start docker --base-url ssh://mydockerhost

Environment

(prefect) λ prefect diagnostics { "config_overrides": { "backend": true, "server": { "endpoint": true, "graphql": { "host": true }, "host": true, "host_port": true, "port": true, "ui": { "endpoint": true, "graphql_url": true, "host": true, "host_port": true, "port": true } } }, "env_vars": [], "system_information": { "platform": "Windows-10-10.0.18362-SP0", "prefect_version": "0.11.5", "python_version": "3.7.0" } }