Update builder implementation to 0.1 Builder Spec

[jromero]

Description

The builder has recently been spec'd via RFC (official spec upcoming).

Proposed solution

• Ensure all environment variables and labels on image are present and correct
• Validate builder API.
  • If value is missing, assume API 0.1 with proper necessary changes to make it compatible.

[dfreilich]

In order to be compliant with the specification, we need to make the following changes:

Env

• The image config's Env field has the environment variable CNB_APP_DIR set to the application directory of the build environment
• The image config's Env field has the environment variable CNB_LAYERS_DIR set to a directory in the builder
• The image config's Env field has the environment variable CNB_PLATFORM_DIR set to a directory in the builder
• The image config's Env field has the environment variable CNB_PLATFORM_DIR set to a directory in the builder
• The image config's Env field has the environment variable SERVICE_BINDING_ROOT set to a directory in the builder, if any services are bound

New Labels

• The image config's Labels field has the label io.buildpacks.builder.api, set to 0.1
• The image config's Labels field has the label io.buildpacks.lifecycle.version, set to the lifecycle version (greater than 0.9.0)
• The image config's Labels field has the label io.buildpacks.lifecycle.apis, set to a JSON object representing the lifecycle apis (taken from what we currently set within the io.buildpacks.builder.metadata)

The io.buildpacks.lifecycle.apis data should look like:

{
  "buildpack": {
    "deprecated": ["<list of versions>"],
    "supported": ["<list of versions>"]
  },
  "platform": {
    "deprecated": ["<list of versions>"],
    "supported": ["<list of versions>"]
  }
}

Changes to Existing Labels

• We should change our current io.buildpacks.builder.metadata labels to match the RFC metadata field, by removing the lifecycle top-level key (and all sub information)

Additional Requirements

We should follow all of the spec recommendations for what a build image should look like, which would require us also setting:

• The image config's Labels field has the label io.buildpacks.stack.maintainer set to the name of the stack maintainer.
• The image config's Labels field has the label io.buildpacks.stack.homepage set to the homepage of the stack.
• The image config's Labels field has the label io.buildpacks.stack.distro.name set to the name of the stack's OS distro.
• The image config's Labels field has the label io.buildpacks.stack.distro.version set to the version of the stack's OS distro.
• The image config's Labels field has the label io.buildpacks.stack.released set to the release date of the stack.
• The image config's Labels field has the label io.buildpacks.stack.description set to the description of the stack.
• The image config's Labels field has the label io.buildpacks.stack.metadata set to additional metadata related to the stack.

[jromero]

Waiting for spec PR buildpacks/spec#193 to be approved.