decorrelate source and target directories

Signed-off-by: Xavier FRANCOIS <xavier.francois@orange.com>

text/0000-decorrelate-source-targer-dirs.md

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+# Meta
+[meta]: #meta
+- Name: Decorrelate source dans target directories
+- Start Date: 2023-04-03
+- Author(s): @xfrancois
+- Status: Draft <!-- Acceptable values: Draft, Approved, On Hold, Superseded -->
+- RFC Pull Request: (leave blank)
+- CNB Pull Request: (leave blank)
+- CNB Issue: [Can't make final image directory different from the CNB_APP_DIR](https://github.com/buildpacks/lifecycle/issues/1034)
+- Supersedes: N/A
+
+# Summary
+[summary]: #summary
+
+Allow CNB users to build image from one directory and have the final image directory structure to be something else. Currently it's possible to change the default target directory (/workspace) by setting the `CNB_APP_DIR` environement variable, but the source directory must also be the one specified in `CNB_APP_DIR`. This behavior is not well suited for constrained environements, like CI.
+
+# Definitions
+[definitions]: #definitions
+
+N/A
+
+# Motivation
+[motivation]: #motivation
+
+- Why should we do this?
+
+We should do this to provide more flexibility for end-users.
+
+- What use cases does it support?
+
+Building from a CI tool, for example Gitlab-CI. We can't chose the source directory from which we are building. On Gitlab-CI the Git folder is checkout on `/builds/<group>>/<project>`. By having a way to decorrelate source and target directories, we could chose the finaly directory structure we want.
+
+- What is the expected outcome?
+
+Support of a new environment variable / parameter that allows to set the source directory. We probably should document that  `CNB_APP_DIR` only handles the target directory.
+
+| Variable           | Default value | Description                |
+|--------------------|---------------|----------------------------|
+| `CNB_APP_DIR`       | `/workspace`    | Target directory structure |
+| `CNB_SOURCE_APP_DIR` | `$CNB_APP_DIR`  | Source directory structure |
+
+# What it is
+[what-it-is]: #what-it-is
+
+This provides a high level overview of the feature.
+
+- Define any new terminology.
+- Define the target persona: buildpack author, buildpack user, platform operator, platform implementor, and/or project contributor.
+- Explaining the feature largely in terms of examples.
+- If applicable, provide sample error messages, deprecation warnings, or migration guidance.
+- If applicable, describe the differences between teaching this to existing users and new users.
+
+# How it Works
+[how-it-works]: #how-it-works
+
+In the Gitlab-CI example, I can't chose the source directory but I want to be able to chose the target directory. I will set the variables like this :
+
+`CNB_SOURCE_APP_DIR`: /builds/group/project
+
+`CNB_APP_DIR`: /workspace
+
+The final image will be available
+
+# Migration
+[migration]: #migration
+
+N/A
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+N/A
+
+# Alternatives
+[alternatives]: #alternatives
+
+- What is the impact of not doing this?
+
+The impact is that people wanting to use CNB on CI environment will have to make some workaround (like copying source folders in another location) in order to have a proper directory structure in the final image.
+
+# Prior Art
+[prior-art]: #prior-art
+
+Discuss prior art, both the good and bad.
+
+# Unresolved Questions
+[unresolved-questions]: #unresolved-questions
+
+- What parts of the design do you expect to be resolved before this gets merged?
+- What parts of the design do you expect to be resolved through implementation of the feature?
+- What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?
+
+# Spec. Changes (OPTIONAL)
+[spec-changes]: #spec-changes
+Does this RFC entail any proposed changes to the core specifications or extensions? If so, please document changes here.
+Examples of a spec. change might be new lifecycle flags, new `buildpack.toml` fields, new fields in the buildpackage label, etc.
+This section is not intended to be binding, but as discussion of an RFC unfolds, if spec changes are necessary, they should be documented here.
+
+# History
+[history]: #history
+
+<!--
+## Amended
+### Meta
+[meta-1]: #meta-1
+- Name: (fill in the amendment name: Variable Rename)
+- Start Date: (fill in today's date: YYYY-MM-DD)
+- Author(s): (Github usernames)
+- Amendment Pull Request: (leave blank)
+
+### Summary
+
+A brief description of the changes.
+
+### Motivation
+
+Why was this amendment necessary?
+--->