Attach to a container's log_stream before they're started

So we're not displaying output of all previous logs for a container, we attach,
if possible, to a container before the container is started.

LogPrinter checks if a container has a log_stream already attached and
print from that rather than always attempting to attach one itself.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

compose/container.py

@@ -19,6 +19,7 @@ class Container(object):
         self.client = client
         self.dictionary = dictionary
         self.has_been_inspected = has_been_inspected
+        self.log_stream = None
 
     @classmethod
     def from_ps(cls, client, dictionary, **kwargs):
@@ -146,6 +147,13 @@ class Container(object):
         log_type = self.log_driver
         return not log_type or log_type == 'json-file'
 
+    def attach_log_stream(self):
+        """A log stream can only be attached if the container uses a json-file
+        log driver.
+        """
+        if self.has_api_logs:
+            self.log_stream = self.attach(stdout=True, stderr=True, stream=True)
+
     def get(self, key):
         """Return a value from the container or None if the value is not set.