docker: cache vendor to speed up docker build (#10296)

Author: siddontang

Dockerfile

@@ -11,10 +11,17 @@ RUN apk add --no-cache \
 RUN wget -O /usr/local/bin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.2/dumb-init_1.2.2_amd64 \
  && chmod +x /usr/local/bin/dumb-init
 
-COPY . /go/src/github.com/pingcap/tidb
+RUN mkdir -p /go/src/github.com/pingcap/tidb
+WORKDIR /go/src/github.com/pingcap/tidb
 
-WORKDIR /go/src/github.com/pingcap/tidb/
+# Cache dependencies
+COPY go.mod .
+COPY go.sum .
 
+RUN GO111MODULE=on go mod download
+
+# Build real binaries
+COPY . .
 RUN make
 
 # Executable image
@@ -27,4 +34,4 @@ WORKDIR /
 
 EXPOSE 4000
 
-ENTRYPOINT ["/usr/local/bin/dumb-init", "/tidb-server"]
+ENTRYPOINT ["/usr/local/bin/dumb-init", "/tidb-server"]