Three pins, two values: Dockerfile said 3.4.5, Dockerfile-export said
3.4.7, .ruby-version said 3.4.5 — the export image had already been
bumped on its own and nobody noticed the other two lagging. Both
Dockerfiles carry a comment telling you to keep them in step.
3.4.10 is the current 3.4.x. CI resolves its Ruby from .ruby-version,
so this is the version the tests now run under.