build: remove warning for multi-platform iidfile

[tonistiigi]

Afaics this should have been removed when the feature was implemented. I tested that it works with regular multi-arch builds as well as with multi-node builds.

@crazy-max

Signed-off-by: Tonis Tiigi tonistiigi@gmail.com

[crazy-max]

@tonistiigi After testing 0.4.2 with the following command:

docker buildx build \
  --iidfile ./iidfile \
  --platform linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/386,linux/ppc64le,linux/s390x
  -t 7zip --file Dockerfile .

iidfile is created but empty. I think this is caused by:

buildx/build/build.go

Line 574 in fb7b670

if len(res) == 1 {

If so I think we should explode containerimage.digest with \n separator and prefix with platform like:

linux/amd64;sha256:99b0eb4a5b89cc9323e289776a0785de78293289107f87bd0785982c45c34524
linux/arm64;sha256:6b1cdd6aa977cc02fac165ae86ad4b7568bf349e750e68b9ca3c217b9d4afbc1
linux/386;sha256:67ef0f98ab23f988735f68d98c955c686f40062b9f5436e63c2e22386c048a3d

[tonistiigi]

@crazy-max you need to make a build that actually creates an image, eg -o type=image or --push or -o type=oci . If you don't see "exporting to image" in the output you also can't have the manifest digest.