build: update Docker versions to Go 1.15.7

This updates the Docker files to the latest minor release of Go which
included some security fixes.

docker/btcd/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.12-alpine as builder
+FROM golang:1.15.7-alpine as builder
 
 LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 

docker/ltcd/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.12-alpine as builder
+FROM golang:1.15.7-alpine as builder
 
 LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"