use alpine 3.12 everywhere

more details here: https://alpinelinux.org/posts/Alpine-3.12.0-released.html

Dockerfile.amd64

@@ -1,4 +1,4 @@
-FROM alpine:3.11
+FROM alpine:3.12
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.arm

@@ -1,4 +1,4 @@
-FROM arm32v6/alpine
+FROM arm32v6/alpine:3.12
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.arm64

@@ -1,4 +1,4 @@
-FROM arm64v8/alpine
+FROM arm64v8/alpine:3.12
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.ppc64le

@@ -1,4 +1,4 @@
-FROM ppc64le/alpine
+FROM ppc64le/alpine:3.12
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.s390x

@@ -1,4 +1,4 @@
-FROM s390x/alpine
+FROM s390x/alpine:3.12
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

images/iperf3/Makefile

@@ -5,13 +5,13 @@ ARCH ?= amd64
 TEMP_DIR := $(shell mktemp -d)
 
 ifeq ($(ARCH),amd64)
-        BASEIMAGE=alpine:3.6
+        BASEIMAGE=alpine:3.12
 endif
 ifeq ($(ARCH),arm64)
-	BASEIMAGE=aarch64/alpine:3.6
+	BASEIMAGE=aarch64/alpine:3.12
 endif
 ifeq ($(ARCH),ppc64le)
-	BASEIMAGE=ppc64le/alpine:3.6
+	BASEIMAGE=ppc64le/alpine:3.12
 endif
 
 all: container