ci: cache buildx job

The current buildx job is taking 60 minutes. This is due to lack of
caching. This commit tries to enable caching as defined in https://docs.docker.com/build/ci/github-actions/cache/

.github/workflows/docker.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - master
-  # pull_request:
 
 permissions:
   contents: read
@@ -42,3 +41,5 @@ jobs:
           push: true
           platforms: linux/amd64,linux/arm64
           tags: steelscheme/steel:latest
+          cache-from: type=registry,ref=steelscheme/steel:latest
+          cache-to: type=inline