[dock] Add logging information to lable based discover

This will show information similar to the describe based
implementation.

beat/core/dock.py

@@ -401,13 +401,17 @@ class Host(object):
 
                 image_info = _parse_image_info(image)
                 key = "{} {}".format(image_info["name"], image_info["version"])
+                image_name = image_info["image"]
 
                 if key in environments:
                     if _must_replace(key, image, environments):
                         environments[key] = image_info
+                        logger.info("Updated '%s' -> '%s'", key, image_name)
                 else:
                     environments[key] = image_info
-                    Host.images_cache[image_info["image"]] = environments[key]
+                    Host.images_cache[image_name] = environments[key]
+                    logger.info("Registered '%s' -> '%s'", key, image_name)
+
             return environments
 
         client = docker.from_env()
@@ -418,6 +422,12 @@ class Host(object):
         executors = client.images.list(filters={"label": ["beat.env.type=execution"]})
         environments = _process_image_list(executors)
 
+        logger.debug(
+            "Found %d environments and %d database environments",
+            len(environments),
+            len(db_environments),
+        )
+
         return environments, db_environments
 
     def create_container(self, image, command):