[Feature Request]: from-args should support docker manifest like syntax

[ajaykmis]

First of all, Thanks for the tool!!

I am trying to use this to create/push a manifest from inside the docker container (can't use docker). I want to avoid writing a temporary file to pass in from-spec.

so, for the from-args, would it be possible to add multiple args (similar to docker manifest) to add images to the manifest. Currently the --template only takes the name of the image, for example, gcr.io/imagename-ARCH, but it wouldn't work if the image names don't conform to a common format.

for example, I build arm64 and amd64 builds separately, and so the tag's are actually different:

arm64: gcr.io/image-name-arm64:14
amd64: gcr.io/image-name-amd64:15 

so, in this case, you can't use the --template args and have to use from-spec to specify the full name for each arch type.
docker manifest command:

docker manifest create gcr.io/image-name:1-multi --amend gcr.io/image-name:1-arm64 --amend gcr.io/image-name/:1-amd64

would be nice to have something like:

./manifest-tool push --from-args --image-name gcr.io/image-name:1-multi  --amend gcr.io/image-name:1-arm64 --amend gcr.io/image-name/:1-amd64

Please let me know if there's another way to do this from the command line.

[YAMLcase]

This would be really useful to rebuild official docker images with only the os/arch we need, i.e. those that we're actually using and passed vulnerability scanning. What we're currently doing is manifest inspect to get select digests, then build a spec file with those digests.:

image: my-registry.example.com/library/busybox:1
manifests:
  -
    image: my-registry.example.com/tmp-repo/busybox@sha256:ccff0c7e8498c0bd8d477777777e1a43b86fb091
    platform:
      architecture: amd64
      os: linux
  -
    image: my-registry.example.com/tmp-repo/busybox@sha256:dca71257cd2e728408888888802146f583486b
    platform:
      architecture: '386'
      os: linux