Deploying to google app engine flexible with go.mod fails because it is looking for dependent libraries in GOPATH

Question

I have the helloworld app from here with a couple of modifications.

1. I've used logrus (to bring in dependency to an external lib)
2. I've added a go.mod using go mod init
3. I have checkout this code outside of my GOPATH.

Everything works on localhost. I'm able to see the hello message on localhost:8080. When I attempt to do a gcloud app deploy I get the following error:

staging for go1.11
Staging Flex app: failed analyzing /home/.../code/app-engine-gomod: cannot find package "github.com/sirupsen/logrus" in any of:
    ($GOROOT not set)
    /home/.../go/src/github.com/sirupsen/logrus (from $GOPATH)
GOPATH: /home/.../go

I've tried with and without go mod vendor doesn't help. What am I doing wrong?