Docker build successfully:
```docker build .
   Sending build context to Docker daemon   84.8MB
   Step 1/10 : FROM golang:1.9-alpine as builder
   1.9-alpine: Pulling from library/golang
   605ce1bd3f31: Pull complete
   d1e3a1512603: Pull complete
   64455c1aaa47: Pull complete
   cdc57526dc05: Pull complete
   92b355b184ea: Pull complete
   5c5463f1346a: Pull complete
   Digest: sha256:42a5e73c830a2379e8f75b195cd53e43cb1c221ac1d75b91265ef0974de6482a
   Status: Downloaded newer image for golang:1.9-alpine
    ---> a4c7ad5b9041
   Step 2/10 : RUN apk add --no-cache     make     git
    ---> Running in 400795a9e773
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/main/x86_64/APKINDEX.tar.gz
   fetch http://dl-cdn.alpinelinux.org/alpine/v3.6/community/x86_64/APKINDEX.tar.gz
   (1/6) Installing libssh2 (1.8.0-r1)
   (2/6) Installing libcurl (7.57.0-r0)
   (3/6) Installing expat (2.2.0-r1)
   (4/6) Installing pcre (8.41-r0)
   (5/6) Installing git (2.13.5-r0)
   (6/6) Installing make (4.2.1-r0)
   Executing busybox-1.26.2-r9.trigger
   OK: 25 MiB in 18 packages
   Removing intermediate container 400795a9e773
    ---> 02ad96bc3da0
   Step 3/10 : COPY . /go/src/github.com/pingcap/tidb
    ---> 08fa94416742
   Step 4/10 : WORKDIR /go/src/github.com/pingcap/tidb/
   Removing intermediate container a850a8ead739
    ---> d8b1af371946
   Step 5/10 : RUN make
    ---> Running in 4c92bc149091
   make parser
   make[1]: Entering directory '/go/src/github.com/pingcap/tidb'
   CGO_ENABLED=0 go build  -o bin/goyacc parser/goyacc/main.go
   bin/goyacc -o /dev/null parser/parser.y
   Parse table entries: 611181 of 1601340, x 16 bits == 1222362 bytes
   bin/goyacc -o parser/parser.go parser/parser.y 2>&1 | egrep "(shift|reduce)/reduce" | awk '{print} END {if (NR > 0) {print "Find conflict in parser.y. Please check y.output for more information."; exit 1;}}'
   rm -f y.output
   make[1]: Leaving directory '/go/src/github.com/pingcap/tidb'
   CGO_ENABLED=0 go build   -ldflags '-X "github.com/pingcap/tidb/mysql.TiDBReleaseVersion=v1.1.0-alpha.1-24-g96bd4e11-dirty" -X "github.com/pingcap/tidb/util/printer.TiDBBuildTS=2018-01-26 11:57:54" -X "github.com/pingcap/tidb/util/printer.TiDBGitHash=96bd4e11621665ec40934b35f1cee7b77aa8a851" -X "github.com/pingcap/tidb/util/printer.TiDBGitBranch=master" -X "github.com/pingcap/tidb/util/printer.GoVersion=go version go1.9.3 linux/amd64"' -o bin/tidb-server tidb-server/main.go
   Build TiDB Server successfully!
   Removing intermediate container 4c92bc149091
    ---> 16da7786d02c
   Step 6/10 : FROM scratch
    --->
   Step 7/10 : COPY --from=builder /go/src/github.com/pingcap/tidb/bin/tidb-server /tidb-server
    ---> 455347e6b0c2
   Step 8/10 : WORKDIR /
   Removing intermediate container 3f4572ccf0bc
    ---> 01bf74f5c71f
   Step 9/10 : EXPOSE 4000
    ---> Running in 23eaa5cc2adc
   Removing intermediate container 23eaa5cc2adc
    ---> fc91019cc0a5
   Step 10/10 : ENTRYPOINT ["/tidb-server"]
    ---> Running in 0773ce0ba0a1
   Removing intermediate container 0773ce0ba0a1
    ---> aaa9c01eaaf9
   Successfully built aaa9c01eaaf9
```

- cleanup empty folders (and _vendor)
- add cleanup commands to `hack/cleanup_vendor.sh`