build-doc action does not need matrix strategy

.github/workflows/build-docs.yaml

@@ -6,15 +6,13 @@ on:
   schedule:
     - cron: "0 0 * * *"
 
+env:
+  STACK: '/cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh'
+  OS: 'ubuntu22'
+
 jobs:
   build-docs:
     runs-on: ubuntu-latest
-    strategy: 
-      fail-fast: false
-      matrix:
-          STACK: ['/cvmfs/sw-nightlies.hsf.org/key4hep/setup.sh']
-          OS: ['alma9',
-               'ubuntu22']
     steps:
     - uses: actions/checkout@v4
     - uses: cvmfs-contrib/github-action-cvmfs@v4
@@ -23,20 +21,19 @@ jobs:
         docker run -it --name CI_container \
                    -v ${GITHUB_WORKSPACE}:/Package \
                    -v /cvmfs:/cvmfs:shared \
-                   -p 80:80 \
-                   -d ghcr.io/key4hep/key4hep-images/${{ matrix.OS }}:latest \
+                   -d ghcr.io/key4hep/key4hep-images/${{ env.OS }}:latest \
                    /bin/bash
     - name: CMake Configure
       run: |
         docker exec CI_container /bin/bash -c 'cd Package; \
            mkdir -p build install; \
-           source ${{ matrix.STACK }}; \
+           source ${{ env.STACK }}; \
            cd build; \
            cmake -DCMAKE_INSTALL_PREFIX=../install ..'
     - name: Generate doxygen documentation
       run: |
         docker exec CI_container /bin/bash -c 'cd Package; \
-           source ${{ matrix.STACK }}; \
+           source ${{ env.STACK }}; \
            cd build; \
            make doc'
     - name: Deploy to GitHub Pages