Newer version of dockers on new installs create OCI format images which ECR doesn't seem to support with the immutability flag on. This is because of buildx running by default on new installs.

The behaviour can be fixed by disabling buildx by adding this config for the daemon:

{
  "features": {
    "buildkit": false
  }
}

another option would be to use podman instead of docker. podman is, from my understanding, mostly a drop-in replacement of docker to build images and by default build images in the application/vnd.docker.distribution.manifest.v2+json format, which is what ECR wants for immutability. Another advantage of podman is that it doesn't require a deamon nor root access. it's also open source for real.

I suggest you add support for podman if the executable is present on the system. The build flags are the same as docker. Eventually i'd like to see the project embrace podman as the default in the docs since you're an open source advocate :)