chore(rootfs/Dockerfile): update go toolchain to 1.15.8 (#268)

See https://github.com/golang/go/issues?q=milestone%3AGo1.15.8+label%3ACherryPickApproved

Author: mboersma

rootfs/Dockerfile

@@ -7,7 +7,7 @@ ENV ANSIBLE_VERSION=2.10.3 \
     AZCLI_VERSION=2.17.1 \
     PYJWT_VERSION=1.7.1 \
     DOCKER_VERSION=20.10.2 \
-    GO_VERSION=1.15.7 \
+    GO_VERSION=1.15.8 \
     GLIDE_VERSION=v0.13.3 \
     GLIDE_HOME=/root \
     HELM_VERSION=3.5.0 \