Publish the y.js Docker image and set up CI for future releases #3106

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

User Story

As an administrator, I want an officially published collaboration server image so that I can deploy collaborative editing without building the image myself.

User Value

Operability, adoption

Acceptance Criteria

• The y.js image (services/yjs) is published to the OpenCloud registry (Docker: opencloudeu/yjs, quay.io: quay.io/opencloudeu/yjs)
• Tags follow the org conventions (semver release tags plus a rolling tag)
• Initial version is 1.0.0-rc.1; the y.js image doesn't mirror the web or backend version but rather has its own version
• Multi-arch build (linux/amd64, linux/arm64)
• A CI workflow builds and pushes on release/tag
• There is a script in /dev/scripts for publishing a new y.js image
• All supported environment variables and their defaults are documented (in README.md)