Multicall launcher

[ekcasey]

RFC 0045
RFC PR: buildpacks/rfcs#84
Spec Issue: buildpacks/spec#105
Spec PR: buildpacks/spec#118

Behavior

When Platform API >= 0.4:

exporter

• export layer with /cnb/process/<type> symlinks
• always export launcher to /cnb/lifecycle/launcher, not <launcher>
• If the default process-type matches a process type
  • set the ENTRYPOINT to the default process symlink
• Else
  • If there is exactly one process type
    - set the ENTRYPOINT to the only process symlink
  • Else set the ENTRYPOINT to /cnb/lifecycle/launcher
• prepend /cnb/process to the PATH
• don't set CNB_PROCESS_TYPE on app image

launcher

• Select process type using $0
• If a process type is selected, treat launcher arguments as additional args to the process type