How to test golang code on concourse not using copy command?

In Concourse I used topflighttech/go-testing docker container to test my-go-api. I find that the simplest solution is just copy src code to /go/src/my-go-api and test it. But I wondering how we could test on concourse straightforward without copy to /go/src/myapi? So the image could stay smaller.

Of course mv is not working. Here is output from Concourse.

+ mv my-go-api /go/src
mv: can't remove 'my-go-api': Resource busy