ci: Enable apt caching for container builds

The Dockerfile is already prepared and works locally. Now also make use
of that on github.

Signed-off-by: Jan Kiszka <[email protected]>

.github/actions/docker-init/action.yml

@@ -35,3 +35,27 @@ runs:
       run: |
         git clone . /home/runner/kas-clone
       shell: bash
+    - name: Cache var-cache-apt
+      id: cache-var-cache-apt
+      uses: actions/cache@v4
+      with:
+        path: var-cache-apt
+        key: var-cache-apt-${{ env.DEBIAN_TAG }}
+    - name: Cache var-lib-apt
+      id: cache-var-lib-apt
+      uses: actions/cache@v4
+      with:
+        path: var-lib-apt
+        key: var-lib-apt-${{ env.DEBIAN_TAG }}
+    - name: Inject var-cache-apt into docker
+      uses: reproducible-containers/[email protected]
+      with:
+        cache-source: var-cache-apt
+        cache-target: /var/cache/apt
+        skip-extraction: ${{ steps.cache-var-cache-apt.outputs.cache-hit }}
+    - name: Inject var-lib-apt into docker
+      uses: reproducible-containers/[email protected]
+      with:
+        cache-source: var-lib-apt
+        cache-target: /var/lib/apt
+        skip-extraction: ${{ steps.cache-var-lib-apt.outputs.cache-hit }}