Bump GitHub Actions to latest major versions

- setup-buildx-action: v2 -> v3
- login-action: v2 -> v3
- build-push-action: v5 -> v6

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

Author: mattgodbolt

.github/workflows/build.yml

@@ -34,15 +34,15 @@ jobs:
         - jakt
     steps:
     - name: Docker Setup Buildx
-      uses: docker/setup-buildx-action@v2
+      uses: docker/setup-buildx-action@v3
     - name: Docker Login
-      uses: docker/login-action@v2
+      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@v5
+      uses: docker/build-push-action@v6
       with:
         push: ${{ github.ref == 'refs/heads/main' && 'true' || 'false' }}
         tags: compilerexplorer/${{ matrix.image }}-builder:latest
@@ -59,15 +59,15 @@ jobs:
         - heaptrack
     steps:
       - name: Docker Setup Buildx
-        uses: docker/setup-buildx-action@v2
+        uses: docker/setup-buildx-action@v3
       - name: Docker Login
-        uses: docker/login-action@v2
+        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@v5
+        uses: docker/build-push-action@v6
         with:
           push: ${{ github.ref == 'refs/heads/main' && 'true' || 'false' }}
           tags: compilerexplorer/${{ matrix.image }}-builder:latest-arm64