Fix ZULIP_GIT_REF to be a string, not a YAML float.

Signed-off-by: Anders Kaseorg <[email protected]>

Author: andersk

docker-compose.yml

@@ -54,7 +54,7 @@ services:
       args:
         # Change these if you want to build zulip from a different repo/branch
         ZULIP_GIT_URL: https://github.com/zulip/zulip.git
-        ZULIP_GIT_REF: 3.3
+        ZULIP_GIT_REF: '3.3'
         # Set this up if you plan to use your own CA certificate bundle for building
         # CUSTOM_CA_CERTIFICATES:
     ports: