ci: move go test into Dockerfile build stage

The runner runs jobs via act + DinD; `docker run -v "$PWD:/src"` from
inside the job container mounts the runner-job filesystem, not the
docker daemon's host fs, so the mount appears empty and `go test ./...`
fails with "directory prefix . does not contain main module".

Run tests in the same container that builds — same workspace, no mount.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/main.yaml

@@ -9,11 +9,6 @@ jobs:
       - name: Check out repo
         uses: actions/checkout@v4
 
-      - name: Run unit tests
-        run: |
-          docker run --rm -v "$PWD:/src" -w /src golang:1.26-alpine \
-            sh -c "go test ./..."
-
       - name: Log in to Gitea registry
         uses: docker/login-action@v3
         with: