Switch to compliant base image

Use compliant base image according to our tech rules of play:
- KDP-1: use an approved Docker base image and version from the list of
    allowed Docker Base Images under the pierone namespace ‘library’.

Signed-off-by: Jonathan Juares Beber <jonathanbeber@gmail.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM registry.opensource.zalan.do/stups/alpine:latest
+FROM registry.opensource.zalan.do/library/alpine-3.12:latest
 MAINTAINER Team Teapot @ Zalando SE <team-teapot@zalando.de>
 
 # add binary