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

Adds max/glacier-rehydration-ux to deploy-s3-proxy.yaml's on.push.branches so
CI builds an s3-proxy image with the x-amz-restore Expose-Headers fix. 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/**'