Resolve "Update golang version in vendored Dockerfile template"

changelogs/unreleased/42917_golang_vendor_templates.yml

+---
+title: Update golang version in vendored Dockerfile template
+merge_request: 42917
+author:
+type: added

vendor/Dockerfile/Golang-alpine.Dockerfile

-FROM golang:1.8-alpine AS builder
+FROM golang:1.15-alpine AS builder
 
 WORKDIR /usr/src/app
 

vendor/Dockerfile/Golang-scratch.Dockerfile

-FROM golang:1.8-alpine AS builder
+FROM golang:1.15-alpine AS builder
 
 # We'll likely need to add SSL root certificates
 RUN apk --no-cache add ca-certificates

vendor/Dockerfile/Golang.Dockerfile

-FROM golang:1.8 AS builder
+FROM golang:1.15 AS builder
 
 WORKDIR /usr/src/app