docker-compose cannot find package

I'm writing a simple app in GO and I have this folder structure

The docker-compose.yml file content is:

version: '2'
services:
  db:
    image: rethinkdb:latest
    ports:
      - "38080:8080"
      - "38015:28015"
      - "39015:29015"
  api:
    image: golang:1.8-alpine
    volumes:
      - .:/go/src/test_server/
    working_dir: /go/src/test_server
    command: go run server.go
    container_name: test_server
    ports:
      - "8085:8085"
    links:
      - db
    tty: true

Everytime I run docker-compose up I receive this error message:

test_server | controllers/users.go:4:3: cannot find package "_/go/src/test_server/vendor/github.com/gin-gonic/gin" in any of: test_server |
/usr/local/go/src/_/go/src/test_server/vendor/github.com/gin-gonic/gin (from $GOROOT) test_server |
/go/src/_/go/src/test_server/vendor/github.com/gin-gonic/gin (from $GOPATH)

It's referring to the controllers package. I'm using github.com/kardianos/govendor to vendor my packages. Do you know what's going on?