testing: fix race condition reading data from containers

Example of a race caught by the race detector:

```
==================
WARNING: DATA RACE
Write at 0x00c4201dea68 by goroutine 374:
  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:726 +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 0x00c4201dea68 by goroutine 357:
  reflect.Value.Bool()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/reflect/value.go:248 +0x52
  encoding/json.isEmptyValue()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:309 +0xd4
  encoding/json.(*structEncoder).encode()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:626 +0x31f
  encoding/json.(*structEncoder).(encoding/json.encode)-fm()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:651 +0x7b
  encoding/json.(*structEncoder).encode()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:637 +0x2e2
  encoding/json.(*structEncoder).(encoding/json.encode)-fm()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:651 +0x7b
  encoding/json.(*ptrEncoder).encode()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:778 +0x12a
  encoding/json.(*ptrEncoder).(encoding/json.encode)-fm()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:783 +0x7b
  encoding/json.(*encodeState).reflectValue()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:323 +0x93
  encoding/json.(*encodeState).marshal()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/encode.go:296 +0xbc
  encoding/json.(*Encoder).Encode()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/encoding/json/stream.go:193 +0xda
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).inspectContainer()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:569 +0x289
  github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.(*DockerServer).(github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing.inspectContainer)-fm()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/fsouza/go-dockerclient/testing/server.go:165 +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 374 (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 357 (finished) 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
==================
```

testing/server.go

@@ -547,10 +547,10 @@ func (s *DockerServer) renameContainer(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusNotFound)
 		return
 	}
-	copy := *container
-	copy.Name = r.URL.Query().Get("name")
 	s.cMut.Lock()
 	defer s.cMut.Unlock()
+	copy := *container
+	copy.Name = r.URL.Query().Get("name")
 	if s.containers[index].ID == copy.ID {
 		s.containers[index] = &copy
 	}
@@ -566,6 +566,8 @@ func (s *DockerServer) inspectContainer(w http.ResponseWriter, r *http.Request)
 	}
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusOK)
+	s.cMut.RLock()
+	defer s.cMut.RUnlock()
 	json.NewEncoder(w).Encode(container)
 }
 
@@ -820,16 +822,18 @@ func (s *DockerServer) waitContainer(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusNotFound)
 		return
 	}
+	var exitCode int
 	for {
 		time.Sleep(1e6)
 		s.cMut.RLock()
 		if !container.State.Running {
+			exitCode = container.State.ExitCode
 			s.cMut.RUnlock()
 			break
 		}
 		s.cMut.RUnlock()
 	}
-	result := map[string]int{"StatusCode": container.State.ExitCode}
+	result := map[string]int{"StatusCode": exitCode}
 	json.NewEncoder(w).Encode(result)
 }
 
@@ -1153,7 +1157,9 @@ func (s *DockerServer) createExecContainer(w http.ResponseWriter, r *http.Reques
 	}
 
 	execID := s.generateID()
+	s.cMut.Lock()
 	container.ExecIDs = append(container.ExecIDs, execID)
+	s.cMut.Unlock()
 
 	exec := docker.ExecInspect{
 		ID:          execID,