Add riscv64 support

[mengzhuo]

Proposed changes

Since riscv64 had be adopted in alpine 3.20 (stable, released in May 22), we can add riscv64 and update alpine.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• [x] I have read the CONTRIBUTING document
• [x] I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• [ ] If applicable, I have added tests that prove my fix is effective or that my feature works
• [ ] If applicable, I have checked that any relevant tests pass after adding my changes
• [x] I have updated any relevant documentation

[mmoll]

@mengzhuo after the latest merges, only the riscv64 addition should remain after a rebase

[mengzhuo]

@mmoll Thanks for your kind reminder, done.

[thresheek]

Thanks, I plan to add it soon. Can you confirm those dockerfiles compile fine on riscv64 hardware? I don't have access to it.

[mengzhuo]

@thresheek Sure! Here is logs

root@visionfive2-2:~/docker-nginx/stable/alpine-slim# uname -a
Linux visionfive2-2 6.8.0-40-generic #40.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Jul 31 06:23:15 UTC 2024 riscv64 riscv64 riscv64 GNU/Linux
root@visionfive2-2:~/docker-nginx/stable/alpine-slim# podman build -t nginx:alpine-slim .
...build log truncated...
root@visionfive2-2:~/docker-nginx/stable/alpine-slim# podman image inspect nginx:alpine-slim | grep Architecture
          "Architecture": "riscv64",
root@visionfive2-2:~/docker-nginx/stable/alpine-slim# podman run -d -p 80:80 --rm nginx:alpine-slim

curl localhost works fine.

[mmoll]

@thresheek Gentle nudge...

[thresheek]

Thanks!

O-I library PR submitted as well: https://github.com/docker-library/official-images/pull/18005