I have my Dockerfile in the root of directory with src/myapp folder, myapp contains myapp.go with main package.
src/myapp
myapp
myapp.go
Dockerfi
myapp needs to be in /go/src/myapp as suggested, or in /usr/local/go/src/myapp. You can add it in ADD section.
/go/src/myapp
/usr/local/go/src/myapp
ADD