chore: ci: cache container build to GHA

This is deliberately applied only to the push-pr workflow, and not the
release workflow, to prevent potential [cache
poisoning](https://docs.zizmor.sh/audits/#cache-poisoning).

Author: nadiamoe

.github/workflows/push-pr.yaml

@@ -59,6 +59,8 @@ jobs:
           push: false
           tags: ci.local/crocochrome:${{ github.sha }}
           platforms: linux/amd64,linux/arm64
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
 
   golangci:
     name: Lint