Docker golang包导入错误：导入路径不以hostname开头

我正在尝试测试docker并去项目 . 这是我的dockerfile

FROM golang

ARG app_env
ENV APP_ENV $app_env

COPY ./ /go/src/github.com/user/myProject/app
WORKDIR /go/src/github.com/user/myProject/app



RUN go get ./
RUN go build

CMD if [ ${APP_ENV} = production ]; \
then \
app; \
else \
go get github.com/pilu/fresh && \
fresh; \
fi  
EXPOSE 8080

它运行良好 . 然后我在我的go程序中添加了一个包“testpack” .

package main

import(
"fmt"
"time"
"testpack"
)

var now = time.Now()
var election = time.Date(2016, time.November, 8, 0, 0, 0, 0, time.UTC)
func main() {
 //get duration between election date and now
tillElection := election.Sub(now)
//get duration in nanoseconds
toNanoseconds := tillElection.Nanoseconds()
//calculate hours from toNanoseconds
hours := toNanoseconds/3600000000000
remainder := toNanoseconds%3600000000000
//derive minutes from remainder of hours
minutes := remainder/60000000000
remainder = remainder%60000000000
//derive seconds from remainder of minutes
seconds := remainder/1000000000 
//calculate days and get hours left from remainder
days := hours/24
hoursLeft := hours%24

fmt.Printf("\nHow long until the 2016 U.S. Presidential election?\n\n%v Days %v Hours %v Minutes %v Seconds\n\n", days, hoursLeft, minutes, seconds)

}

现在我跑了=> docker build ./

我收到了一个错误

package testpack：无法识别的导入路径“testpack”（导入路径不以hostname开头）

任何帮助表示赞赏 .