Add support for docker run --tmpfs flag.

Signed-off-by: Philip Walls <pawalls@rabidgeek.com>

docs/compose-file.md

@@ -226,6 +226,15 @@ Custom DNS search domains. Can be a single value or a list.
       - dc1.example.com
       - dc2.example.com
 
+### tmpfs
+
+Mount a temporary file system inside the container. Can be a single value or a list.
+
+    tmpfs: /run
+    tmpfs:
+      - /run
+      - /tmp
+
 ### entrypoint
 
 Override the default entrypoint.

docs/extends.md

@@ -302,8 +302,8 @@ replaces the old value.
 > This is because `build` and `image` cannot be used together in a version 1
 > file.
 
-For the **multi-value options** `ports`, `expose`, `external_links`, `dns` and
-`dns_search`, Compose concatenates both sets of values:
+For the **multi-value options** `ports`, `expose`, `external_links`, `dns`,
+`dns_search`, and `tmpfs`, Compose concatenates both sets of values:
 
     # original service
     expose: