Add note in README pointing to community fork continuing image updates

[adrianjagielak]

The existing IMPORTANT block says image updates stop on May 1st 2026. Anyone who lands on the README after that date is going to look for where the same image is still being built. This PR adds two sentences to the same block pointing them at a community-maintained fork I've stood up.

The fork mirrors the build pipeline (same sdk/Dockerfile, same tag scheme: stable, beta, exact versions like 3.41.9, multi-arch on GHCR), so for most users it's a drop-in registry swap (ghcr.io/cirruslabs/flutter -> ghcr.io/adrianjagielak/flutter).

A few notes to make this easy to merge or reject:

• No endorsement implied: The note explicitly states the fork is independent and not affiliated with or endorsed by Cirrus Labs or OpenAI. Happy to reword if you'd prefer different framing (e.g. "third-party fork", "unaffiliated fork", or just dropping the mention entirely).
• No ask: I'm not requesting a transfer, a redirect, ownership of the GHCR package, or anything beyond this README pointer. I'll maintain the fork on a best-effort, volunteer basis.
• Easy to revert: If you change your mind later, it's a one-commit revert.

I understand if the answer is "no thanks"; in that case feel free to close. Posting it because I think it saves users a search, and the existing notice is the natural place to point them. Thanks for years of maintaining these images!

[fkorotkov]

Probably this is a better new home for the images fluttercommunity/community#177. Let them know if you'd like to maintain it there.

[dbebawy]

Thanks for setting this up, @adrianjagielak. We maintain One Coptic Library, a Flutter app shipping to iOS/Android/macOS/Windows/Linux/web, and just migrated our CI base images from ghcr.io/cirruslabs/flutter to your fork — clean drop-in swap, and it's already ahead of upstream (you've published 3.44.1, which cirruslabs never did).

I'd be glad to help keep this maintained going forward — whether here or under a Flutter Community–org home if @fkorotkov's suggestion lands. Happy to pitch in on the build/CI pipeline, review PRs, or help keep version tags current. Just let me know where it'd be most useful.