feature request: support "docker load" like functionality for input caching

[eedwards-sk]

while there is support for build caches already, this doesn't solve a use case where you want to start with layers from a previous image build without having a local build cache

ideally, I would be able to provide an input image.tar file as an input cache

this image would then be loaded before the build starts, so any cached layers (with no changes) are not rebuilt

similar to a "docker load", except since this task library doesn't really deal with repositories, that could be handled by an upstream resource that saves the image.tar file and passes it to this task

not sure if this is possible based on a cursory glance at img itself, but I wanted to toss out this use case either way

[jbhannah]

I ran into this too, so I made a builder that uses BuildKit directly and takes advantage of the ability to save the build cache either locally or to a registry. The script within the image itself is more complex but the usage of it is largely the same (and it's building itself!).

[vito]

@jbhannah Interesting! Maybe we should just switch to using buildkit directly?

@cirocosta also has a buildkit task here: https://github.com/cirocosta/buildkit-task

[vito]

@jbhannah I went ahead and wrote a new task which uses buildkit directly under the hood: https://github.com/vito/oci-build-task - I had been wanting to rewrite it with tests and in Go so that was a good centerpiece for the refactor. 🙂

@eedwards-sk I'm going to archive this repo, so I'll close this issue. The equivalent feature request on the new task is here: concourse/oci-build-task#1