Add reusable workflow for setting up SSH agent

.github/workflows/stage.yml

@@ -1,10 +1,6 @@
 name: Deploy (Staging)
 
-on:
-  workflow_dispatch
-
-env:
-  SSH_AUTH_SOCK: /tmp/ssh-agent.sock
+on: workflow_dispatch
 
 jobs:
   stage:
@@ -17,6 +13,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set up SSH agent
+        uses: ./.github/actions/ssh
+        with:
+          config: ${{ vars.SSH_CONFIG }}
+          key: ${{ secrets.SSH_PRIVATE_KEY }}
+          known-hosts: ${{ vars.SSH_KNOWN_HOSTS }}
+
       - name: Set up Java
         uses: actions/setup-java@v4
         with:
@@ -29,14 +32,6 @@ jobs:
       - name: Build with Gradle
         run: ./gradlew installDist
 
-      - name: Set up SSH agent
-        run: |
-          ssh-agent -a $SSH_AUTH_SOCK > /dev/null
-          ssh-add - <<< "${{ secrets.SSH_PRIVATE_KEY }}"
-          mkdir -p ~/.ssh
-          echo '${{ vars.SSH_CONFIG }}' > ~/.ssh/config
-          echo '${{ vars.KNOWN_HOSTS }}' > ~/.ssh/known_hosts
-
       - name: Publish to staging server
         run: |
           rsync build/install/helma/ antville.dev:/ \