Update docker.io/mccutchen/go-httpbin Docker tag to v2.18.3 (#5452)

Author: openverse-bot

api/compose.yml

@@ -64,6 +64,6 @@ services:
       - api
     # Use go-httpbin because it's only 15 MB compared to upstream HTTPBin's 204 MB
     # That helps reduce the network burden of spinning up a development environment
-    image: docker.io/mccutchen/go-httpbin:2.18.1
+    image: docker.io/mccutchen/go-httpbin:2.18.3
     expose:
       - "8080"