jobs/registry-update-base-images: one image per line

Make the list of images more git friendly, and sort the debian releases
by date.

Related: OS#4969
Change-Id: Ic724aa4b2c71aa398ad7f7049e8f3406a3883eaa

jobs/registry-update-base-images.yml

@@ -16,7 +16,17 @@
     builders:
       - shell: |
           REGISTRY="registry.osmocom.org"
-          IMAGES="debian:stretch debian:buster debian:jessie debian:sid ubuntu:zesty centos:centos7 centos:centos8"
+          IMAGES="
+            debian:jessie
+            debian:stretch
+            debian:buster
+            debian:sid
+
+            ubuntu:zesty
+
+            centos:centos7
+            centos:centos8
+          "
           for i in $IMAGES; do
             src=$i
             dst=${REGISTRY}/$i