Next-gen replacement for buildstep

[stuartpb]

After going over the issues for buildstep, I've come to some conclusions:

• Maintaining a list of buildpacks as part of the image is some stone bullshit.
• stack/builder is pretty much the only meaningful component - the rest should be handled by the use case, ie. part of a proper Dockerfile, or a volume for buildpacks (see below).
• stack/builder direly needs some maintenance, some merging with https://github.com/flynn/flynn/blob/master/slugbuilder/builder/build.sh
  • should be a little less heavy on the things it puts into /app - especially not deleting and clobbering it (as not doing so would let /app be mounted so output could go into a volume).
• The right way to get the proper Cedar environment would be to start with https://github.com/progrium/cedarish
  • This also fixes the locale issues of buildstep

So, anyway, here's the planned future version of this:

• Fork stack/builder into something. I might do it as stuartpb/packbuilder. (might also just make a light Docker build that's cedarish + the packbuilder builder.)
• Have buildpacks be (numbered if needed) dirs-in-a-dir (like plugins) manged by a plugin (plushu-buildpacks) (which retrieves with install / update hooks plushu-git#9's hooks), and have meta-plugins for getting stuff like the base Heroku buildpack set (or Dokku / buildstep's buildpack set, as included with progrium/buildstep).
• Mount packbuild/builder into a cedarish environment (as a volume, I guess), along with mounted buildsteps, then have it build like that (let's stick with the image-creating plan for now, using the same way as how we do buildstep).

[stuartpb]

see also progrium/buildstep#109

[stuartpb]

Continuing discussion in plushu/plushu-build-cedarish#1