ci(s3-proxy): build s3-proxy image for glacier dev stack (TEMP)

Re-add max/glacier-rehydration-ux to deploy-s3-proxy.yaml's on.push.branches
so CI rebuilds the s3-proxy image at the current tip, which now carries both
the x-amz-restore Expose-Headers fix and the merged-in nginx 1.30.2 bump.
Revert before merge.

Author: fiskus

.github/workflows/deploy-s3-proxy.yaml

@@ -4,6 +4,8 @@ on:
   push:
     branches:
       - master
+      # TEMP: build s3-proxy image for the glacier dev stack; revert before merge.
+      - max/glacier-rehydration-ux
     paths:
       - .github/workflows/deploy-s3-proxy.yaml
       - 's3-proxy/**'