Why the file created in dockerfile RUN not on host, I already mount host dir for service

I am using docker-compose to run my golang app.

Here is my Dockerfile

FROM golang:1.13
COPY go.mod ./
RUN go mod download
COPY . .
RUN go build -o main .
CMD ["/app/main"]

and my docker-compose.yml

version: '3.7'
        build: ./myapp
        container_name: myapp
            - ./myapp:/app

When I run docker-compose build The main file not appear on myapp dir.

docker-compose up myapp not work, because main file not found.
But docker run mypp can work. How can i build main.go in dockerfile and stay the main in my host?

Source: StackOverflow