add docker caching

Author: untitaker

.github/workflows/build-image.yml

@@ -41,3 +41,5 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=registry,ref=ghcr.io/untitaker/mastodon-list-bot:latest
+          cache-to: type=inline