Example of data race found:

```
==================
WARNING: DATA RACE
Write at 0x00c420df4468 by goroutine 559:
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).stopContainer()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:724 +0x15a
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).(github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.stopContainer)-fm()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:169 +0x5f
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).handlerWrapper.func1()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:379 +0x70a
  net/http.HandlerFunc.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1918 +0x51
  github.com/tsuru/tsuru/vendor/github.com/gorilla/mux.(*Router).ServeHTTP()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/gorilla/mux/mux.go:133 +0x18a
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).ServeHTTP()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:340 +0x271
  net/http.serverHandler.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2619 +0xbc
  net/http.(*conn).serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1801 +0x83b
Previous read at 0x00c420df4468 by goroutine 165:
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).attachContainer()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:790 +0x356
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).(github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.attachContainer)-fm()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:174 +0x5f
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).handlerWrapper.func1()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:379 +0x70a
  net/http.HandlerFunc.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1918 +0x51
  github.com/tsuru/tsuru/vendor/github.com/gorilla/mux.(*Router).ServeHTTP()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/gorilla/mux/mux.go:133 +0x18a
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).ServeHTTP()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:340 +0x271
  net/http.serverHandler.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2619 +0xbc
  net/http.(*conn).serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1801 +0x83b
Goroutine 559 (running) created at:
  net/http.(*Server).Serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2720 +0x37c
  net/http.Serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2323 +0xe2
Goroutine 165 (running) created at:
  net/http.(*Server).Serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2720 +0x37c
  net/http.Serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2323 +0xe2
==================
```