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.

Dockerfile

@@ -4,7 +4,7 @@
 # /make/builder.Dockerfile
 # /.github/workflows/main.yml
 # /.github/workflows/release.yml
-FROM golang:1.15.6-alpine as builder
+FROM golang:1.15.7-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

@@ -4,7 +4,7 @@
 # /make/builder.Dockerfile
 # /.github/workflows/main.yml
 # /.github/workflows/release.yml
-FROM golang:1.15.6-alpine as builder
+FROM golang:1.15.7-alpine as builder
 
 LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 

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>"
 

make/builder.Dockerfile

@@ -4,7 +4,7 @@
 # /dev.Dockerfile
 # /.github/workflows/main.yml
 # /.github/workflows/release.yml
-FROM golang:1.15.6-buster
+FROM golang:1.15.7-buster
 
 MAINTAINER Olaoluwa Osuntokun <laolu@lightning.engineering>