enabled image build cache on gha (#5)

Author: olwig

.github/workflows/ghcr.yml

@@ -1,10 +1,11 @@
 name: Build and push to GHCR
+
 on:
   push:
     branches: [main]
     paths:
       - Containerfile
-
+      - .github/workflows/**
   workflow_dispatch:
 
 jobs:
@@ -20,6 +21,8 @@ jobs:
         id: vars
         run: echo "timestamp=$(date -u +'%Y%m%d-%H%M%S')" >> "$GITHUB_OUTPUT"
 
+      - uses: docker/setup-buildx-action@v3
+
       - uses: docker/login-action@v3
         with:
           registry: ghcr.io
@@ -31,6 +34,8 @@ jobs:
           context: .
           file: Containerfile
           push: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
           tags: |
-            ghcr.io/${{ github.repository }}:${{ steps.vars.outputs.timestamp }}
             ghcr.io/${{ github.repository }}:latest
+            ghcr.io/${{ github.repository }}:${{ steps.vars.outputs.timestamp }}