Adds stack descriptor RFC

text/stacks/0003-stack-descriptor.md

@@ -0,0 +1,83 @@
+# Stack Descriptor and Tooling
+
+## Summary
+
+The stack-building codebase,
+[`create-stack`](https://github.com/paketo-buildpacks/stacks/tree/main/create-stack)
+should be moved into the `jam` tooling CLI, and allow users to configure their
+stack build via a common configuration file.
+
+## Motivation
+
+Today, `create-stack` is highly coupled to the `bionic` stacks we currently
+support. This makes it difficult to support alternative stacks and platforms or
+to allow the codebase to be reused by external parties for developing their own
+stack.
+
+## Detailed Explanation
+
+The `create-stack` codebase will be refactored to allow users to build stacks
+using a configuration file with the following format:
+
+```toml
+id = "<stack id>"                # required
+homepage = "<homepage uri>"      # optional
+maintainer = "<maintainer name>" # optional
+
+include-legacy-sbom = true # optional; default = false; enables https://github.com/paketo-buildpacks/rfcs/blob/main/text/stacks/0001-stack-package-metadata.md
+
+platforms = ["linux/amd64", "linux/arm64", "<other-platform>"] # optional; default = ["linux/amd64"]; choices defined in https://docs.docker.com/desktop/multi-arch/
+
+[distro]
+  name = "<os distro name>"       # optional
+  version = "os distro version>"  # optional
+
+[build]
+  tag = "<image tag>"                 # required
+  dockerfile = "<path to dockerfile>" # required
+  description = "<image description>" # optional
+
+  uid = 1001                   # required
+  gid = 1000                   # required
+  shell = "<user login shell>" # optional; default = "/sbin/nologin"
+
+  [build.args] # optional
+    example-arg = "<example-value>"
+
+[run]
+  tag = "<image tag>"                 # required
+  dockerfile = "<path to dockerfile>" # required
+  description = "<image description>" # optional
+
+  uid = 1002                   # required
+  gid = 1000                   # required
+  shell = "<user login shell>" # optional; default = "/sbin/nologin"
+
+  [run.args] # optional
+    example-arg = "<example-value>"
+```
+
+Then the codebase will be merged into the `jam` CLI under a `create-stack`
+command. Once complete, a user should be able to build a stack with a given
+descriptor file, `stack.toml`, using the following command:
+
+`jam create-stack --config stack.toml`
+
+## Rationale and Alternatives
+
+We could abandon the `create-stack` codebase entirely and expect that anyone
+developing stacks just build a `Dockerfile` following the
+[directions](https://buildpacks.io/docs/operator-guide/create-a-stack/)
+outlined in the buildpacks documentation.
+
+## Implementation
+
+The `create-stack` command will be refactored to support this new configuration
+file format and the features it allows. The codebase will then be moved into
+the `jam` codebase as a command within that tool. This aligns more with how the
+project maintains its tooling and allows us to keep tooling consolidated under
+the tooling subteam.
+
+## Prior Art
+
+The buildpacks spec defines several descriptor files in TOML format.