Caching/Resume builds

[amouat]

It can take a long time to run a build, especially for larger projects. Often writing a melange file will require making changes and tweaks to later stages, it would be great to be able to restart a build from a previous point that having to restart from scratch.

[rawlingsj]

fwiw I've been using a local version of a wolfi SDK which helps when building melange configs. I think @jdolitsky has a branch that is better than my local; version, we could probably try and get that added to https://github.com/chainguard-images/sdk. While it's not a resume feature it does seem to be an easier way to get feedback when creating melange configs when you rely on docker to melange build...