docs: clarify tag usage in build action

.github/workflows/build.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
     inputs:
       tag_custom:
-        description: 'A custom docker image tag e.g. "next"'
+        description: 'A custom docker image tag. Use "next" to deploy to designsafeci-next.tacc.utexas.edu. Use "dev" (or, in the future, "pprd", to deploy to pprd.designsafe-ci.org).'
         required: false
         type: string
   push: