Spike: Lifecycle should run hooks’ bin/build

[natalieparellano]

Broken out from #709

(4) Lifecycle should run hooks’ bin/build

Hooks may be executed in parallel
For each hook:

• <layers> is replaced by <output>
• If a hook is missing bin/build, the hook root should be treated as a pre-populated directory
• Output files should be validated:
  • build.toml:
    • May include args table
  • launch.toml:
    • May include args table
    • Must not include processes or slices

(5) Lifecycle should provide Dockerfiles output by hooks to subsequent phases

The files should probably be copied to a volume (/layers? Does this depend on buildpacks/rfcs#145 ?)

[cmoulliard]

Hooks

That should b great to have a sample about what a hooked project will look like (+ Dockerfile, Hook TOML, ...)

1. Non hooked

builders
└── maven-jvm
    └── builder.toml
 
buildpacks/maven
├── bin
│   ├── build
│   └──detect
└── buildpack.toml

2. Hooked project
   ???

[natalieparellano]

Completed as part of #709 (see comment).