build error github.com/shurcooL/highlight_go on go 1.10 alpine (avelino#2291)

ref avelino#1903

```
github.com/shurcooL/highlight_go (download)
github.com/sourcegraph/annotate (download)
github.com/sourcegraph/syntaxhighlight (download)
github.com/shurcooL/highlight_go/internal/go/token
src/github.com/shurcooL/highlight_go/internal/go/token/token.go:12: syntax error: unexpected = in type declaration
github.com/sourcegraph/annotate
github.com/sourcegraph/syntaxhighlight
ERROR: Service 'webhook' failed to build: The command '/bin/sh -c go get -u -v github.com/shurcooL/highlight_go' returned a non-zero code: 2
```

Author: avelino

Dockerfile

@@ -1,7 +1,7 @@
-FROM golang:alpine
+FROM golang:1.10-alpine
 
 RUN apk add --update -t build-deps curl go git libc-dev gcc libgcc
-RUN go get -u -v github.com/shurcooL/github_flavored_markdown/... github.com/gorilla/mux
+RUN go get -u -v github.com/shurcooL/github_flavored_markdown github.com/gorilla/mux
 
 WORKDIR /srv