The documentation on cloning is wrong.
It says to use the alpine:git image for cloning. This image doesn't exist.

Also, a go build won't work by default on golang image because the drone workspace is on /drone/src volume and the GOPATH in the golang image points to /go. So, it clones a Go project to /drone/src which is outside GOPATH. And the build will fail because it can't get the deps right.
I guess the solution would be to have a Go project using modules. I haven't tried yet.