Update used Go version to 1.15.x
Overview
We're currently on 1.13.8, while the recent stable version of Go is 1.15.4. And as we're slowly approaching Go 1.16 to be released, we're risking - AGAIN - to use an unsupported version of Go.
Why we are blocked
As we seen in jobs like https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/682053228 and https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/682095024 we are facing issue compiling so s390x
on a newer version of Go because we are using an older version of docker. We need gitlab-org/ci-cd/docker-machine#14 (closed) to update our shared runner fleet so that we can use a newer version of Docker that will solve the issue.
Edited by Steve Xuereb