Solve security alerts connected to Alpine

Upgrade to latest Alpine (3.14) to solve busybox-related security
warnings that are causing the build to fail.

Remove the note about performance issues associated with Alpine 3.13 or
newer as these were no longer possible to prove.

Ref:
crystal-lang/distribution-scripts#127 (comment)

docker/1.2/Dockerfile

@@ -2,10 +2,7 @@
 
 # ---
 # 1. Use Alpine as base
-#
-# NOTE: Use Alpine 3.12 as newer version present a performance regression
-# Ref: https://github.com/crystal-lang/distribution-scripts/pull/127#issuecomment-920489700
-FROM alpine:3.12 AS base
+FROM alpine:3.14 AS base
 
 # ---
 # 2. Upgrade system and installed dependencies for security patches