ci: use alpine mirror for gitea image build

Dockerfile.gitea

@@ -20,7 +20,8 @@ COPY backend/internal/web ./backend/internal/web
 RUN cd frontend && pnpm run build
 
 FROM ${GO_IMAGE} AS backend-builder
-RUN apk add --no-cache ca-certificates git
+RUN sed -i 's#https://dl-cdn.alpinelinux.org/alpine#https://mirrors.tencent.com/alpine#g' /etc/apk/repositories && \
+    apk add --no-cache ca-certificates git
 WORKDIR /src
 
 COPY backend/go.mod backend/go.sum ./backend/
@@ -48,7 +49,8 @@ LABEL maintainer="Wei-Shaw <github.com/Wei-Shaw>"
 LABEL description="Sub2API - AI API Gateway Platform"
 LABEL org.opencontainers.image.source="http://git.jianshixingqiu.com/kgod/sub2api"
 
-RUN apk add --no-cache \
+RUN sed -i 's#https://dl-cdn.alpinelinux.org/alpine#https://mirrors.tencent.com/alpine#g' /etc/apk/repositories && \
+    apk add --no-cache \
     ca-certificates \
     tzdata \
     curl \