Bake complains about result only in build cache, even if it's used

[ciaranmcnulty]

We have an old site that builds a deployment asset and I'm containerising it.

I defined 2 targets (with different Dockerfiles); one that spits out the deployable package onto disk to fit into the old deployment method, and another that builds a web container consuming that package:

target "build" {
    # [...]
    output = [
        "./build",
    ]
}

target "web" {
    # [...]
    contexts = {
        deploy-package = "target:build"
    }
    tags = [
        "myorg/web",
    ]
}

When I build the web target:

docker buildx build web --load --push

Because the build target is built I get this confusing mix of messages in the log:

 => => exporting layers                                                                                                                                                0.0s
 => => exporting manifest sha256:d6c864e28edcf8d0efc9439b16e64348b6cdc9fc16cdbe98556a1acdaa1ff960                                                                      0.0s
 => => exporting config sha256:e8dfdb36d627783b7e6a457043ac5fca6c364d90b29444e56780d962228c5c19                                                                        0.0s
 => => sending tarball                                                                                                                                                11.4s
 => importing to docker                                                                                                                                                0.0s
WARNING: No output specified for docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load

Building the deploy target directly doesn't warn (because it has a file output)

I think the mechanism that triggers a build because of the additional build context should be suppressing that warning.

[crazy-max]

@ciaranmcnulty Yes this is a known issue, see #968