Configurable Previous Image

[ekcasey]

Description

pack build should accept am optional --previous-image flag that allows users to set the previous image to a particular tag reference, digest reference, or (when performing a daemon build) image ID.

This is useful If I want to rebuild from a particular previous version of my app. Currently I would need to tag the previous image with the tag to which I want to export to force this behavior. This is an extra and potentially undesirable step.

Proposed solution

Usage: pack build --previous-image <prev-image> <image>

Lifecycle Arguments

When the builder is trusted, pack should invoke creator with the -previous-image flag or CNB_PREVIOUS_IMAGE env var.

creator -previous-image <prev-image> <image>

When the builder is not truster, pack should pass the previous image to analyzer

analyzer <prev-image>

Edge Cases

When pack build is called with --publish, <prev-image> must be in the same image registry as <image> (exporter currently does not support previous images in a different registry).

Until we have a better solution for managing caches volumes this feature will only be useful for reusing launch = true, cache = false layers, because the previous cache is not necessarily provided.

Additional context

• This feature should be documented somewhere

[micahyoung]

This feels a bit like docker build's --cache-from option, which really speeds up rebuilding Windows images in particular.

[sambhav]

does pack build --cache-image implement the required functionality for this? In which case should this be closed?

[ekcasey]

@samj1912 some layer metadata comes from the previous app image, rather than from the cache image.

How these options interact might be a bit confusing for users but I think it is worth exposing this useful knobs now (maybe as an experimental feature?), and we can brainstorm a more cohesive UX for future iterations.

[importhuman]

Hello! I'd like to work on this. Can you give me some pointers as to where I should start? I know some Go, and the basics of Docker and how to use the pack CLI.

[jromero]

Hi @importhuman, very excited to hear that this is something you are interested in contributing.

Here are a few steps I can think off:

Prerequisites:

1. Read our DEVELOPMENT.md document and install all the prerequisites.
2. Make sure you can run make test and all tests pass.

Development:

1. Create a unit test for analyze phase within here.
2. Implement changes for analyze phase here.
3. Follow steps 1-2 for create (aka creator) [tests and implementation].
4. Add new flag previous-image here.
5. Create a set of unit tests similar to cache-image with requirements detailed in issue description.
6. Create an acceptance test similar to cache-image.

Hope this helps. If you run into any issues along the way feel free to reach out either here or in slack.