CI (Alpine Linux): Use ccache

The compilation (especially of the C++ objects) is pretty slow with qemu.
Use ccache to speed up build time on subsequent runs.

.github/workflows/emulated.yml

@@ -38,30 +38,75 @@ jobs:
           packages: >
             bash
             build-base
+            ccache
             cmake
             gfortran
             eigen-dev
             lapack-dev
 
+      - name: prepare ccache
+        # create key with human readable timestamp
+        # used in action/cache/restore and action/cache/save steps
+        id: ccache-prepare
+        run: |
+          echo "key=ccache:alpine:${{ matrix.arch }}:${{ github.ref }}:$(date +"%Y-%m-%d_%H-%M-%S"):${{ github.sha }}" >> $GITHUB_OUTPUT
+
+      - name: restore ccache
+        # setup the GitHub cache used to maintain the ccache from one job to the next
+        uses: actions/cache/restore@v4
+        with:
+          # location of the ccache of the chroot in the root file system
+          path: /home/runner/rootfs/alpine-latest-${{ matrix.arch }}/home/runner/.ccache
+          key: ${{ steps.ccache-prepare.outputs.key }}
+          # Prefer caches from the same branch. Fall back to caches from the default branch.
+          restore-keys: |
+            ccache:alpine:${{ matrix.arch }}:${{ github.ref }}
+            ccache:alpine:${{ matrix.arch }}
+
+      - name: configure ccache
+        env:
+          CCACHE_MAX: ${{ matrix.ccache-max }}
+        run: |
+          test -d ~/.ccache || mkdir ~/.ccache
+          echo "max_size = 20M" >> ~/.ccache/ccache.conf
+          echo "compression = true" >> ~/.ccache/ccache.conf
+          ccache -s
+          which ccache
+
       - name: configure
         run: |
           echo "gcc --version"
           gcc --version
           echo "gcc -dumpmachine"
           gcc -dumpmachine
+          echo " "
           mkdir -p ${GITHUB_WORKSPACE}/build && cd ${GITHUB_WORKSPACE}/build
           cmake \
             -DEXAMPLES=ON \
             -DMPI=OFF \
             -DICB=ON \
             -DEIGEN=ON \
+            -DCMAKE_C_COMPILER_LAUNCHER="ccache" \
+            -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
             ..
 
       - name: build
         run: |
           cd ${GITHUB_WORKSPACE}/build
           cmake --build .
 
+      - name: ccache status
+        continue-on-error: true
+        run: ccache -s
+
+      - name: save ccache
+        # Save the cache after we are done (successfully) building
+        # This helps to retain the ccache even if the subsequent steps are failing.
+        uses: actions/cache/save@v4
+        with:
+          path: /home/runner/rootfs/alpine-latest-${{ matrix.arch }}/home/runner/.ccache
+          key: ${{ steps.ccache-prepare.outputs.key }}
+
       - name: test
         run: |
           cd ${GITHUB_WORKSPACE}/build