Drop Node.js v5, add NODE_ENV ARG

Support for Node.js v5 was dropped at the end of June of this year and
is no longer being updated.

See: https://nodejs.org/en/blog/community/v5-to-v7/

This also adds NODE_ENV ARG to the onbuild template so you can do
things like `docker build --build-arg NODE_ENV=production`

library/node

@@ -48,22 +48,6 @@ argon-slim: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd
 4-wheezy: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 4.5/wheezy
 argon-wheezy: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 4.5/wheezy
 
-5.12.0: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12
-5.12: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12
-5: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12
-
-5.12.0-onbuild: git://github.com/nodejs/docker-node@50b56d39a236fd519eda2231757aa2173e270807 5.12/onbuild
-5.12-onbuild: git://github.com/nodejs/docker-node@50b56d39a236fd519eda2231757aa2173e270807 5.12/onbuild
-5-onbuild: git://github.com/nodejs/docker-node@50b56d39a236fd519eda2231757aa2173e270807 5.12/onbuild
-
-5.12.0-slim: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/slim
-5.12-slim: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/slim
-5-slim: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/slim
-
-5.12.0-wheezy: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/wheezy
-5.12-wheezy: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/wheezy
-5-wheezy: git://github.com/nodejs/docker-node@5ad063e3ba340743e394114d3038d0fd4e0fe570 5.12/wheezy
-
 6.6.0: git://github.com/nodejs/docker-node@62a39d8d527a8992734ba2d066c3983fe560ee44 6.6
 6.6: git://github.com/nodejs/docker-node@62a39d8d527a8992734ba2d066c3983fe560ee44 6.6
 6: git://github.com/nodejs/docker-node@62a39d8d527a8992734ba2d066c3983fe560ee44 6.6