Dockerfile is updated (FROM...AS...) (#10)

This patch fixes a warning of "docker build":

WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line xxx)

Author: dmitry-lipetsk

Dockerfile--alpine.tmpl

@@ -1,13 +1,13 @@
 ARG PYTHON_VERSION 
 
 # --------------------------------------------- base1
-FROM python:${PYTHON_VERSION}-alpine as base1
+FROM python:${PYTHON_VERSION}-alpine AS base1
 
 # --------------------------------------------- base1
-FROM base1 as base2
+FROM base1 AS base2
 
 # --------------------------------------------- final
-FROM base2 as final
+FROM base2 AS final
 
 RUN apk add bash