runtime-ci docker image EOL

CF Runtime

Hey all,

*TL;DR: we have deprecated the runtime-ci Docker image, and will soon
delete it from Docker Hub.*

Thanks to Concourse, it's a cinch to build, maintain, and use Docker images
in your CI pipelines. And it's easy to have clean, slim images with the
minimum requirements for your builds. The release integration team no
longer uses the runtime-ci Dockerfile that lives in cf-release. Some fun
facts about that image:

- it has multiple versions of ruby in it
- it has a .bashrc, so many builds using it would source ~/.bashrc
- it was building spiff from source (master of spiff is unrecognizable
compared to the last official release of spiff, though still magically
backwards compatible)

We'd like to delete the image from Docker Hub. Please let us know if there
are any objections.

CF Release Integration Team
Pivotal Software