GDK-in-a-box: Add LOCALE env vars to GDK-in-a-box container image

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What does this merge request do and why?

This merge request fixes locale encoding issues in the GDK-in-a-box Docker container that cause failures when processing UTF-8 characters (emojis, international characters) in various tools like Danger, Git, and Ruby applications, see #2035 (comment 2785270169) .

The container was generating UTF-8 locales but not setting them as the default, causing processes to fall back to the ASCII-only POSIX locale. This resulted in encoding errors when tools encountered non-ASCII characters, particularly in commit messages and JSON parsing.

🛠️ with ❤️ at Siemens

How to set up and validate locally

1. Build the updated Docker image with the locale fix:

   docker build -t gdk-in-a-box:locale-fix -f support/gdk-in-a-box/Dockerfile .

2. Run a container from the new image:

   docker run -it gdk-in-a-box:locale-fix /bin/bash

3. Verify the locale is correctly set to UTF-8:

   locale
   # Should show LANG=en_US.UTF-8 and LC_ALL=en_US.UTF-8
   
   locale charmap
   # Should show UTF-8, not ANSI_X3.4-1968 or ASCII

4. Test UTF-8 handling with a practical example:

   # Create a file with emoji
   echo "Test 🚀" > test.txt
   cat test.txt
   
   # Test with Ruby
   ruby -e "puts '🤖 UTF-8 works!'"
   
   # Test with Git commit containing emoji
   git init test-repo && cd test-repo
   echo "test" > file.txt
   git add file.txt
   git commit -m "Initial commit 🎉"

5. Run Danger to ensure it can parse emoji without errors:

   bundle exec danger dry_run
   # Should complete without "Invalid byte sequence" errors

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.

Related to #2035