Add workflow to manually build only GCC 16

ChangeLog:

	* .github/workflows/build-gcc16.yml: New file.

Author: mattkretz

.github/workflows/build-gcc16.yml

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+# Copyright © 2025      GSI Helmholtzzentrum fuer Schwerionenforschung GmbH
+#                       Matthias Kretz <[email protected]>
+
+name: Build only GCC 16 Image
+
+on:
+  workflow_dispatch:  # Allows manual trigger via GitHub UI
+
+permissions:
+  contents: read
+  packages: write  # Required to push to GHCR
+
+jobs:
+  gcc16:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile.gcc16
+          push: true
+          tags: ghcr.io/${{ github.repository }}/gcc16