fluentd docker image changes

[repeatedly]

Hi folks,

Sorry for the late annoucement.

First, we recently removed 'onbuild' from default Dockerfile to follow Docker best practice.

https://github.com/fluent/fluentd-docker-image/issues/23

And we separated tags for normal and onbuild images.

The former is a 'latest' and 'v0.12.26'.

The latter has '-onbuild' suffix like 'latest-onbuild' and 'v0.12.26-onbuild'.

Older images use onbuild. So if you need onbuild behvaior,

use 'latest-onbuild' instead of 'latest'.

Second, we don't maintain ubuntu-base build actively.

Alpine Linux seems enough to run for Fluentd.

If you want to run Fluentd on Ubuntu, forking Dockerfile is better.

Again, sorry for the late announcment and confusion.

If you have any quesiton, please let me know.

Thanks,

Masahiro