Update golang version in vendored Dockerfile template

The following discussion from !42169 (merged) should be addressed:

• @splattael started a discussion:

  Non-blocking We might want to update golang to either 1.15 in a followup in one swipe:

  • FROM golang:1.8 AS builder
  • FROM golang:1.8-alpine AS builder
  • FROM golang:1.8-alpine AS builder
  • FROM golang:1.8 AS builder
  • image: golang:1.13
  • image: golang:latest