Simplify CI workflow by replacing setup-go with Docker container using Go 1.26 image

.gitea/workflows/ci.yml

@@ -7,13 +7,11 @@ on:
 jobs:
   build-test:
     runs-on: docker
+    container:
+      image: golang:1.26
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
-      - name: Set up Go
-        uses: actions/setup-go@v6
-        with:
-          go-version: '1.26'
       - name: Run tests with coverage
         run: |
           go test -v -coverprofile=coverage.out -coverpkg=./... ./...