Add support for configurable entrypoints

[dsabeti]

Add support for configurable entrypoints

Summary

Currently, the buildpackapplifecycle always uses exec bash -c ' to start an app. However, we want to allow buildpacks to specify a different entrypoint -- an alternative to bash -c -- for their apps.

This change allows buildpacks to specify an alternative during the supply phase, where the supply script can produce a config.yml that specifies this new entrypoint in a new config stanza:

name: Has Buildpack Config
version: 3.14
config:
  entrypoint_prefix: custom-entrypoint

If the buildpack specifies a custom entrypoint, then the buildpackapplfiecycle will use exec '' to start the application. If the buildpack does not specify a custom entrypoint, bash -c is used as it always was. This change is backwards compatible.

Diego repo

• buildpackapplifecycle

Describe alternatives you've considered (optional)

N/A

Additional Text Output, Screenshots, or contextual information (optional)

PR to buildpackapplifecycle: cloudfoundry/buildpackapplifecycle#58
PR to bump buildpackapplifecycle in diego-release: #706