ci: simplify workflows to use docker CLI only

- remove Node-based actions (checkout, buildx, login, build-push)
- rely on gitea workspace checkout and plain docker commands
- avoid Node requirement on self-hosted runner

.gitea/workflows/pr-validation.yml

@@ -49,9 +49,6 @@ jobs:
   validate-docker:
     runs-on: linux
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
       - name: Validate Dockerfile syntax
         run: |
           if [ -f "Dockerfile" ]; then
@@ -86,22 +83,7 @@ jobs:
   build-test:
     runs-on: linux
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
-      - name: Build Docker image (no push)
-        uses: docker/build-push-action@v5
-        with:
-          context: .
-          push: false
-          tags: karaokepedia:pr-${{ github.event.pull_request.number }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
-
-      - name: Test container starts
+      - name: Build and test Docker image
         run: |
           docker build -t karaokepedia:test .
           CONTAINER_ID=$(docker run -d -p 8080:80 karaokepedia:test)
@@ -145,9 +127,6 @@ jobs:
   check-files:
     runs-on: linux
     steps:
-      - name: Checkout code
-        uses: actions/checkout@v4
-
       - name: Check required files exist
         run: |
           REQUIRED_FILES=(