[openstreetmap/openstreetmap-website] Pull docker ruby version into arg (PR #6354)

[rkoeze]

### Description
Pull the Dockerfile Ruby version in to an ARG so we can override it. This will make it easier to run Ruby 3.3 and 3.4, [which we also support](https://github.com/openstreetmap/openstreetmap-website/blob/master/INSTALL.md#prerequisites). Now building the Dockerfile with a different Ruby version can be done like this:

```
docker-compose build --build-arg RUBY_VERSION=3.3
```

### How has this been tested?
The tests pass and I can run the app locally. I was able to confirm the correct Ruby version is still being pulled after the change by looking at the Docker build logs:

```
 => [web  1/10] FROM docker.io/library/ruby:3.2-bookworm at sha256:4bcc61f192db4d9f72c87998fa77f97deaaeb0c1021951632f4cb986b3c779de                                                                                                               33.8s
```

And this is with the RUBY_VERSION set by the command line:

```
=> [web  1/10] FROM docker.io/library/ruby:3.3-bookworm at sha256:49ee09d4811028286bb9510599999622fb7e6264190ebe645e17b3b078d633a6                                                                                                                3.7s
```
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6354

-- Commit Summary --

  * Pull docker ruby version into arg

-- File Changes --

    M Dockerfile (3)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6354.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6354.diff

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