Update dockerfile (pingcap#8363)

Dockerfile

@@ -1,10 +1,14 @@
 # Builder image
-FROM golang:1.10.1-alpine as builder
+FROM golang:1.11.2-alpine as builder
 
 RUN apk add --no-cache \
+    wget \
     make \
     git
 
+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
 
 WORKDIR /go/src/github.com/pingcap/tidb/
@@ -15,10 +19,13 @@ RUN make
 FROM scratch
 
 COPY --from=builder /go/src/github.com/pingcap/tidb/bin/tidb-server /tidb-server
+COPY --from=builder /usr/local/bin/dumb-init /usr/local/bin/dumb-init
+
+
 
 WORKDIR /
 
 EXPOSE 4000
 
-ENTRYPOINT ["/tidb-server"]
+ENTRYPOINT ["/usr/local/bin/dumb-init", "/tidb-server"]