Upgrade Alpine version

Signed-off-by: Manuel Buil <mbuil@suse.com>

Dockerfile.amd64

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.13
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.arm

@@ -1,4 +1,4 @@
-FROM arm32v6/alpine:3.12
+FROM arm32v6/alpine:3.13
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.arm64

@@ -1,4 +1,4 @@
-FROM arm64v8/alpine:3.12
+FROM arm64v8/alpine:3.13
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.ppc64le

@@ -1,4 +1,4 @@
-FROM ppc64le/alpine:3.12
+FROM ppc64le/alpine:3.13
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"
 

Dockerfile.s390x

@@ -1,4 +1,4 @@
-FROM s390x/alpine:3.12
+FROM s390x/alpine:3.13
 
 LABEL maintainer="Tom Denham <tom@tigera.io>"