docker: update to latest golang base image

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.14.5-alpine as builder
+FROM golang:1.15.6-alpine as builder
 
 # Force Go to use the cgo based DNS resolver. This is required to ensure DNS
 # queries required to connect to linked containers succeed.

dev.Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.13-alpine as builder
+FROM golang:1.15.6-alpine as builder
 
 LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"