Use Alpine 3.15 for rootfs stage

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.11 as rootfs-stage
+FROM alpine:3.15 as rootfs-stage
 
 # environment
 ENV REL=focal

Dockerfile.aarch64

@@ -1,4 +1,4 @@
-FROM alpine:3.11 as rootfs-stage
+FROM alpine:3.15 as rootfs-stage
 
 # environment
 ENV REL=focal

Dockerfile.armhf

@@ -1,4 +1,4 @@
-FROM alpine:3.11 as rootfs-stage
+FROM alpine:3.15 as rootfs-stage
 
 # environment
 ENV REL=focal