I recommend opening an issue with the Docker team for this. As I recall, they did not ship arm or arm64 images because the official install script (https://get.docker.com/) did not support those architectures. This has since changed, so perhaps now they would consider shipping an armv7 image.
EDIT: apparently I created an issue for this a while back. See publish arm32v7 image · Issue #67 · docker-library/docker · GitHub