Need recommended .dockerignore for onbuilds

[yosifkit]

Node, Ruby, Rails

[tianon]

Updated list:

$ bashbrew list --all | grep ':onbuild$'
django:onbuild
golang:onbuild
iojs:onbuild
maven:onbuild
mono:onbuild
node:onbuild
pypy:onbuild
python:onbuild
rails:onbuild
ruby:onbuild

[tianon]

Further updated list........

$ bashbrew list --all | grep ':onbuild$'
clojure:onbuild
django:onbuild
erlang:onbuild
golang:onbuild
iojs:onbuild
maven:onbuild
mono:onbuild
node:onbuild
pypy:onbuild
python:onbuild
rails:onbuild
ruby:onbuild
sentry:onbuild

It might be enough to add something generic about this to .template-helpers/variant-onbuild.md (which already discourages extended use of :onbuild tags).

[yosifkit]

Onbuild images are being deprecated: docker-library/official-images#2076