GH-55402: [CI][Packaging] Use lowecase for repository name for Docker image name (#55403)

### Rationale for this change
GH-55402

### What changes are included in this PR?
send repo names in lower case format in `docker_image` function. This change affects `linux-packages` CI.

### Are these changes tested?
Yes, in fork

### Are there any user-facing changes?
N/A

### Was AI used for this PR?

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**PR code and description written by:**

- [x] Human
- [x] AI

**Reviewed before submission by:**

- [x] Human
- [ ] AI
- [ ] Not reviewed

* GitHub Issue: #55402

Authored-by: Alina (Xi) Li <96995091+alinaliBQ@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: alinaliBQ

dev/tasks/linux-packages/helper.rb

@@ -77,7 +77,8 @@ def github_repository
     end
 
     def docker_image(os, architecture)
-      "ghcr.io/#{github_repository}/package-#{super}"
+      # Docker image repository names must be lowercase
+      "ghcr.io/#{github_repository.downcase}/package-#{super}"
     end
   end
 end