add branch to staging

.drone.yml

@@ -1,6 +1,6 @@
 pipeline:
   modify_staging:
-    image: debian:stretch
+    image: alpine/git
     commands:
       - mv content/_meta-staging.md content/_meta.md
       - sed -i s/{0xDEADBEEF}/$(git rev-parse --short HEAD)/g content/_meta.md