bad signature error and temporary error when apk update from local alpine package repositories

I am beginner of docker compiling. I am going to build docker image with Dockerfie in offline ubuntu mode.
I have tried to build Grafana open source project in offline ubuntu mode. I did download alpine packages from online alpine package repositories. I did setup local alpine package repository and generate signing keys for alpine packages in offline mode. When I build using “make build-docker-full” command in offline ubuntu 24.04, I can’t update apks. I encountered this error.

fetch https://dl-cdn.alpinelinux.org/alpine/v3.20/main/x86_64/APKINDEX.tar.gz
WARNING: updating and opening https://dl-cdn.alpinelinux.org/alpine/v3.20/main: temporary error (try again later)
fetch https://dl-cdn.alpinelinux.org/alpine/v3.20/community/x86_64/APKINDEX.tar.gz
WARNING: updating and opening https://dl-cdn.alpinelinux.org/alpine/v3.20/community: temporary error (try again later)
unavailable, 0 stale: 66 distinct packages available
WARNING: opening file:///opt/apks: BAD signature

How to fix it up? Please thank you very much!