Since we use golang 1.12 and gomod, it is not necessary
to match the impor path to `/go/src/github.com/coredns/coredns`
in order to build coredns. For that reason the build command
inside docker could be simplified with:
```
$ docker run --rm -i -t -v $PWD:/v -w /v golang:1.12 make
```
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>