Add container_name option

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/config.py

@@ -50,6 +50,7 @@ DOCKER_CONFIG_KEYS = [
 
 ALLOWED_KEYS = DOCKER_CONFIG_KEYS + [
     'build',
+    'container_name',
     'dockerfile',
     'expose',
     'external_links',

compose/service.py

@@ -157,6 +157,12 @@ class Service(object):
         - starts containers until there are at least `desired_num` running
         - removes all stopped containers
         """
+        if self.custom_container_name() and desired_num > 1:
+            log.warn('The "%s" service is using the custom container name "%s". '
+                     'Docker requires each container to have a unique name. '
+                     'Remove the custom name to scale the service.'
+                     % (self.name, self.custom_container_name()))
+
         if self.specifies_host_port():
             log.warn('The "%s" service specifies a port on the host. If multiple containers '
                      'for this service are created on a single host, the port will clash.'
@@ -531,7 +537,8 @@ class Service(object):
             for k in DOCKER_CONFIG_KEYS if k in self.options)
         container_options.update(override_options)
 
-        container_options['name'] = self.get_container_name(number, one_off)
+        container_options['name'] = self.custom_container_name() \
+            or self.get_container_name(number, one_off)
 
         if add_config_hash:
             config_hash = self.config_hash()
@@ -703,6 +710,9 @@ class Service(object):
             '{0}={1}'.format(LABEL_ONE_OFF, "True" if one_off else "False")
         ]
 
+    def custom_container_name(self):
+        return self.options.get('container_name')
+
     def specifies_host_port(self):
         for port in self.options.get('ports', []):
             if ':' in str(port):