Enable private signing of CI builds

.github/workflows/docker-publish-released.yml

@@ -6,6 +6,7 @@ on:
     - '*'
 
 env:
+  ASF_PRIVATE_SNK: ${{ secrets.ASF_PRIVATE_SNK }}
   PLATFORMS: linux/amd64,linux/arm,linux/arm64
   TAG: released
 
@@ -35,6 +36,13 @@ jobs:
         username: ${{ secrets.DOCKERHUB_USERNAME }}
         password: ${{ secrets.DOCKERHUB_TOKEN }}
 
+    - name: Prepare private key for signing
+      shell: sh
+      run: |
+          if [ -n "${ASF_PRIVATE_SNK-}" ]; then
+              echo "$ASF_PRIVATE_SNK" | base64 -d > "resources/ArchiSteamFarm.snk"
+          fi
+
     - name: Prepare environment outputs
       shell: sh
       run: |