How to download all dependencies with vgo and a given go.mod?

tl;dr: In current go master and in future go releases, go mod download will do this job. For now you need a hack.

On the gophers slack, I was referred to this issue: https://github.com/golang/go/issues/26610 where I learned that this would do more or less what I want (assuming here that you're using go version 1.11beta3):

# Populate the module cache based on the go.{mod,sum} files.
COPY go.mod .
COPY go.sum .
RUN go list -e $(go list -f '{{.Path}}' -m all)

Later in the thread, Russ Cox implements go mod download, which will be available in the next release of go, allowing me to remove the silly go list hack above.

update: go mod download is now widely available and you should use it on go 1.12 and later