Squashed 'docker/helixdns/' content from commit 9ccefa6

git-subtree-dir: docker/helixdns
git-subtree-split: 9ccefa6e449ea300301b77bb40cb498f447df654
Merge commit '17b2d5fa029c0d9469cd8e71606e68b31888b94a' as 'docker/helixdns'
Squashed 'docker/elk/' content from commit ea5afb6

git-subtree-dir: docker/elk
git-subtree-split: ea5afb63317dceb7b2585bdd04576e43c61353c4
Merge commit 'f139cd437a4e80faca2c3069e8b8e1372ef20ac8' as 'docker/elk'
organize docker-image repositories as git subtrees