* uses COPY instead of ADD following
 https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#add-or-copy
* prints alpine release version during build
* removes redundant commands
* does not touch Dockerfile.arm64 and Dockerfile.armv7 - they are almost
  identical to Dockerfile and we may unify and use a single Dockerfile
  for all builds later.

Followup on #2546

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>