add caching to the build actions

the cache expires in 7 days and is limited to 10GB
https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy

.github/workflows/build.yml

@@ -28,10 +28,14 @@ jobs:
           context: "{{defaultContext}}:nginx"          
           push: true
           tags: ghcr.io/ufal/dockerized-nginx-with-shibboleth/nginx-shib:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
       -
         name: Build and push shibboleth
         uses: docker/build-push-action@v5
         with:
             context: "{{defaultContext}}:shibboleth"
             push: true
-            tags: ghcr.io/ufal/dockerized-nginx-with-shibboleth/shibboleth:latest
+            tags: ghcr.io/ufal/dockerized-nginx-with-shibboleth/shibboleth:latest
+            cache-from: type=gha
+            cache-to: type=gha,mode=max