Add new --pull option in build.

Signed-off-by: Christophe Labouisse <christophe@labouisse.org>

compose/service.py

@@ -700,7 +700,7 @@ class Service(object):
             security_opt=security_opt
         )
 
-    def build(self, no_cache=False):
+    def build(self, no_cache=False, pull=False):
         log.info('Building %s' % self.name)
 
         path = self.options['build']
@@ -714,7 +714,7 @@ class Service(object):
             tag=self.image_name,
             stream=True,
             rm=True,
-            pull=False,
+            pull=pull,
             nocache=no_cache,
             dockerfile=self.options.get('dockerfile', None),
         )