Dockerfile: update Go to 1.15.8

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

Dockerfile

@@ -6,7 +6,7 @@ ARG RUNC_VERSION=1.0.0-rc92
 ARG CNI_PLUGINS_VERSION=0.9.0
 ARG CNI_ISOLATION_VERSION=0.0.3
 ARG BUILDKIT_VERSION=0.8.1
-ARG GO_VERSION=1.15.6
+ARG GO_VERSION=1.15.8
 
 FROM ubuntu:${UBUNTU_VERSION} AS base
 ENV DEBIAN_FRONTEND=noninteractive