Added Alpine 3.15

- removed Alpine 3.12 and 3.13 to keep only two Alpine variants

Author: xmikus01

12/alpine3.13/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.15
 
 ENV NODE_VERSION 12.22.8
 

12/alpine3.12/Dockerfile renamed to 12/alpine3.15/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.15
 
 ENV NODE_VERSION 14.18.2
 

12/alpine3.12/docker-entrypoint.sh renamed to 12/alpine3.15/docker-entrypoint.sh

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.15
 
 ENV NODE_VERSION 16.13.1