Try reading Ruby version from .ruby-version

.gitea/workflows/deploy.yaml

@@ -9,12 +9,17 @@ jobs:
   deploy:
     runs-on: cth-ubuntu-latest
     steps:
+    - name: Read Ruby file
+      id: ruby
+      run: echo "version=$(cat .ruby-version)" >> $GITHUB_OUTPUT
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v3
     - name: Push image
       uses: docker/build-push-action@v6
       with:
         push: true
+        build-args:
+          RUBY_VERSION: ${{ steps.ruby.outputs.version }}
         tags: |
           docker.atomaka.com/budget:latest
           docker.atomaka.com/budget:${{gitea.sha}}