Building Docker image from multi-architecture base

Hi there,

We have pushed a multi-architecture Docker-image to AWS ECR and using it to generate other images. This is how the image and manifests look like on AWS ECR:

 % docker buildx imagetools inspect xxxx.dkr.ecr.us-east-1.amazonaws.com/my-image:2.8.1-pg14
Name:    xxxx.dkr.ecr.us-east-1.amazonaws.com/my-image:2.8.1-pg14
MediaType: application/vnd.docker.distribution.manifest.list.v2+json
Digest:    sha256:59370d981db57776e336a97685c9a3930cf8227cdcbe86e6a19ce5665ab5f455
           
Manifests: 
  Name:   xxxx.dkr.ecr.us-east-1.amazonaws.com/my-image:2.8.1-pg14@sha256:e32ab6eb62bdfa2f639dac598987ef4f3a1557ed6eb459bc319734a540295224
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/arm64
             
  Name:  xxxx.dkr.ecr.us-east-1.amazonaws.com/my-image:2.8.1-pg14@sha256:1f977546f6d8b61a8fdb7d9b78873efa34e643430b8e02a4a223a890a9d771e1
  MediaType: application/vnd.docker.distribution.manifest.v2+json
  Platform:  linux/amd64

Then we have a very simple Dockerfile, which builds the final image for each architecture:

FROM --platform=linux/arm64 xxxx.dkr.ecr.us-east-1.amazonaws.com/my-image:2.8.1-pg14
COPY xxx.txt /etc/conf

The image gets build but the architecture is wrong. AWS ECR seems to ignore the --platform=linux/arm64 attribute in FROM and uses linux/amd64 instead.

Is this not supported on AWS ECR?

Thanks