fix docker go module downloads

backend/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.7
 #
 # Multi-stage build for backend API and worker.
 #
@@ -7,18 +8,31 @@
 #
 
 ARG GO_VERSION=1.24.5
+ARG GOPROXY=https://goproxy.cn,direct
+ARG GOSUMDB=sum.golang.google.cn
 
 FROM golang:${GO_VERSION} AS build
+ARG GOPROXY
+ARG GOSUMDB
+ENV GOPROXY=${GOPROXY} \
+    GOSUMDB=${GOSUMDB}
+
 WORKDIR /src/backend
 
 COPY backend/go.mod backend/go.sum ./
-RUN go mod download
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    go mod download
 
 COPY backend/ ./
 
 ENV CGO_ENABLED=0
-RUN go build -trimpath -ldflags="-s -w" -o /out/api ./cmd
-RUN go build -trimpath -ldflags="-s -w" -o /out/worker ./cmd/worker
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    go build -trimpath -ldflags="-s -w" -o /out/api ./cmd
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    go build -trimpath -ldflags="-s -w" -o /out/worker ./cmd/worker
 
 FROM alpine:3.21 AS base
 RUN apk add --no-cache ca-certificates tzdata && adduser -D -H -s /sbin/nologin app
@@ -35,4 +49,3 @@ ENTRYPOINT ["/app/api"]
 FROM base AS worker
 COPY --from=build /out/worker /app/worker
 ENTRYPOINT ["/app/worker"]
-