Dockerfile: bump to Go 1.16 base images

Dockerfile

@@ -1,5 +1,5 @@
 # Build Geth in a stock Go builder container
-FROM golang:1.15-alpine as builder
+FROM golang:1.16-alpine as builder
 
 RUN apk add --no-cache make gcc musl-dev linux-headers git
 

Dockerfile.alltools

@@ -1,5 +1,5 @@
 # Build Geth in a stock Go builder container
-FROM golang:1.15-alpine as builder
+FROM golang:1.16-alpine as builder
 
 RUN apk add --no-cache make gcc musl-dev linux-headers git