Fix backend Containerfile

Signed-off-by: Asai Neko <sugar@sne.moe>

container/backend.Containerfile

@@ -1,6 +1,7 @@
 FROM docker.io/golang:1.25.5-alpine AS backend-build
 WORKDIR /app
 COPY . /app
+RUN go install github.com/swaggo/swag/cmd/swag@latest
 RUN go mod tidy && \
     go generate . \
     go build -o /app/nixcn-cms