[openstreetmap/openstreetmap-website] Add and unignore `.ruby-version` file to ensure consistent dev Ruby (PR #6333)

[Grant]

Firefishy left a comment (openstreetmap/openstreetmap-website#6333)

My humble recommendation for the container would be to stick to the [Official Ruby upstream containers](https://hub.docker.com/_/ruby). 

Either automatic upgrades of minor version:
```
FROM ruby:3.4-bookworm
```

Or exact version pinned:
```
FROM ruby:3.4.5-bookworm
```

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