Bump GitHub Actions to latest major versions

- setup-buildx-action -> v3
- login-action -> v3
- build-push-action -> v6
- cache-to: add mode=max for better layer caching

Fixes deprecated set-output and save-state warnings.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mattgodbolt

.github/workflows/build.yml

@@ -8,19 +8,19 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - name: Docker Setup Buildx
-      uses: docker/setup-buildx-action@v1
+      uses: docker/setup-buildx-action@v3
     - name: Docker Login
-      uses: docker/login-action@v1
+      uses: docker/login-action@v3
       with:
         username: ${{ secrets.DOCKERHUB_USERNAME }}
         password: ${{ secrets.DOCKERHUB_PASSWORD }}
     - name: Build and push to Docker Hub
       id: hub_build
-      uses: docker/build-push-action@v2
+      uses: docker/build-push-action@v6
       with:
         push: true
         tags: compilerexplorer/ruby-builder:latest
         cache-from: type=registry,ref=compilerexplorer/ruby-builder:latest
-        cache-to: type=inline
+        cache-to: type=inline,mode=max
     - name: Docker Hub Image Digest
       run: echo ${{ steps.hub_build.outputs.digest }}