Dockerfile: use GoLang 1.15
-FROM registry.svc.ci.openshift.org/openshift/release:golang-1.13 AS builder
+FROM registry.svc.ci.openshift.org/openshift/release:golang-1.15 AS builder
WORKDIR /go/src/github.com/openshift/machine-config-operator
COPY . .
# FIXME once we can depend on a new enough host that supports globs for COPY,
