Add tag 3.4.12

[dsw9742]

New stable release is out, and it contains a lot of fixes (including regressions introduced in 3.4.11).
http://zookeeper.apache.org/doc/r3.4.12/releasenotes.html
Let me know if I can help.

[dsw9742]

Thank you for adding the new the new tag. Question for you: its very possible I misunderstand the relationship between this repository and Docker Hub, but should there be a 3.4.12 image available at official Zookeeper if there is a 3.4.12 tag here?

[31z4]

Hey @dsw9742 Sorry for confusion here. The image makes it to the Docker Hub once the corresponding https://github.com/docker-library/official-images PR (docker-library/official-images#4351) is merged. It's been merged just recently.