`onbuild` variant

[zakame]

Several language/toolchain images now have an onbuild variant to help jumpstart derivative images. Might be useful here for Perl especially as the base Perl images are relatively larger than others.

Particular examples might be web framework images derived from this image, which in turn might also use onbuild for the user's own custom app images.

[afirth]

https://github.com/GrahamDumpleton/mod_wsgi-docker does a good job if anyone is looking for a starting point

[zakame]

On a related note, Docker 17.05 now supports multi-stage builds

[tianon]

Probably worth doing a read through docker-library/official-images#2076 -- most images onbuild variants are now officially deprecated, because the flexibility/usability of them just cannot meet a simple Dockerfile example in the image documentation.

[zakame]

@tianon sounds good, I too prefer documenting how to make images with ONBUILD instead of providing a variant.

[zakame]

docker-library/docs#968 just got accepted, thanks @yosifkit @tianon!

I will close this once I confirm seeing the updated docs on the Hub.