build small image from scratch (open: no such file or directory)

I tried to use scratch to build a small image. I turned off the CGO but still fail to read the file when the program runs. I have got the error: "open ./app/a.txt: no such file or directory". Is there other reasons that the program cannot read the file?

FROM golang:alpine AS builder

RUN apk update && apk add --no-cache git

WORKDIR $GOPATH/src/scratch
ADD . .

RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -ldflags '-w -extldflags "-static"' -o /go/bin/scratch

FROM scratch
# Copy static executable.
COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
COPY --from=builder /go/bin/scratch /go/bin/scratch
# Run the binary.
ENTRYPOINT ["/go/bin/scratch"]
func main() {
    resp, err := http.Get("https://google.com")
    check(err)
    body, err := ioutil.ReadAll(resp.Body)
    check(err)
    fmt.Println(len(body))
    LocalFile := "./app/a.txt"
    fmt.Println(LocalFile)

    dat, err := ioutil.ReadFile(LocalFile)
    check(err)
    fmt.Print(string(dat))

    f, err := os.Open(LocalFile)
    check(err)

    b1 := make([]byte, 5)
    n1, err := f.Read(b1)
    check(err)
    fmt.Printf("%d bytes: %sn", n1, string(b1[:n1]))
}
func check(err error) {
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

Source: StackOverflow