User custom image of go on CI server.

This image has some necessary tools for testing.

.drone.yml

@@ -4,7 +4,7 @@ workspace:
 
 pipeline:
   test:
-    image: golang:1.10-alpine
+    image: cjsaylor/go-alpine-sdk:1.10
     commands:
       - go get -u golang.org/x/tools/cmd/cover
       - go test -cover $(go list ./... | grep -v /vendor/)