Dockerfile: redirect the contributor to update.sh

The `*/*/Dockerfile` are created from `Docker*template` by the
update.sh script. We add a comment at the beginning of the file with
information to find their origin. We also make them readonly to remind
people who would like to improve these files that they are not meant
to be modified.

13.0/fpm-alpine/Dockerfile

@@ -1,3 +1,4 @@
+# DO NOT EDIT: created by update.sh from Dockerfile-alpine.template
 FROM php:7.1-fpm-alpine
 
 # entrypoint.sh and cron.sh dependencies