Deprecate docker runtime

The plan is to deprecate it in Stein and remove it in Train.
Podman is now the default.

Change-Id: I43d8f44862591f7b77ee8ab4552dce32428e3701

paunch/runner.py

@@ -30,6 +30,9 @@ class BaseRunner(object):
         # Leverage pre-configured logger
         self.log = log or common.configure_logging(__name__)
         self.cont_log_path = cont_log_path
+        if self.cont_cmd == 'docker':
+            self.log.warning('docker runtime is deprecated in Stein '
+                             'and will be removed in Train.')
 
     @staticmethod
     def execute(cmd, log=None, quiet=False):

releasenotes/notes/docker_deprecate-a12eb9c4ab7e6567.yaml

@@ -0,0 +1,4 @@
+---
+deprecations:
+  - |
+    Docker runtime is deprecated in Stein and will be removed in Train.