fix: update Alpine repository mirror and install build/runtime dependencies

Dockerfile

@@ -47,7 +47,9 @@ ENV GOPROXY=${GOPROXY}
 ENV GOSUMDB=${GOSUMDB}
 
 # Install build dependencies
-RUN apk add --no-cache git ca-certificates tzdata
+RUN sed -i 's|dl-cdn.alpinelinux.org|mirrors.aliyun.com|g' /etc/apk/repositories && \
+    apk update && \
+    apk add --no-cache git ca-certificates tzdata
 
 WORKDIR /app/backend
 
@@ -89,7 +91,9 @@ LABEL description="Sub2API - AI API Gateway Platform"
 LABEL org.opencontainers.image.source="https://github.com/Wei-Shaw/sub2api"
 
 # Install runtime dependencies
-RUN apk add --no-cache \
+RUN sed -i 's|dl-cdn.alpinelinux.org|mirrors.aliyun.com|g' /etc/apk/repositories && \
+    apk update && \
+    apk add --no-cache \
     ca-certificates \
     tzdata \
     su-exec \