perf: add BuildKit cache mounts to speed up Go build in Docker

Mount persistent cache for go build and module directories so
subsequent builds only recompile changed packages.

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Build stage
 FROM golang:1.25-alpine AS builder
 WORKDIR /app
@@ -5,7 +6,9 @@ COPY go.mod go.sum ./
 ENV GOPROXY=https://goproxy.cn,direct
 RUN go mod download
 COPY . .
-RUN CGO_ENABLED=0 go build -o pr-helper .
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg/mod \
+    CGO_ENABLED=0 go build -o pr-helper .
 
 # Runtime stage
 FROM alpine:3.20