use go1.16 alpine docker images for tests

Author: quasilyte

_test/install/binary_gopath/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 ARG release
 COPY . /root

_test/install/binary_nogopath/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 ARG release
 COPY . /root

_test/install/gitclone/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 COPY . /root
 

_test/install/gitclone_nobuildtag/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 COPY . /root
 

_test/install/gitclone_toolsgo/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 COPY . /root
 

_test/regress/issue103/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6-alpine AS build
+FROM golang:1.16-alpine AS build
 
 COPY . /root
 

_test/regress/issue193/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15.6
+FROM golang:1.16-alpine AS build
 
 COPY . /root