Update platform for single version Docker images

What does this MR do and why?

Update the platform for single version Docker images to match the platform for other Docker images in this project.

Results in a update to minify:

• Before: https://pkgs.alpinelinux.org/packages?name=minify&branch=v3.18&repo=&arch=&maintainer=
• After: https://pkgs.alpinelinux.org/packages?name=minify&branch=v3.19&repo=&arch=&maintainer=

Test jobs:

• https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/6466820241
• https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/6466820243

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Run docker buildx build --file dockerfiles/single.Dockerfile --platform=linux/amd64 . --build-arg VER=16.10. Command should complete successfully.
4. Run scripts/minify-assets.sh dest/ public/ (create dest if needed). Command should complete successfully.
5. Check out main branch.
6. Run scripts/minify-assets.sh dest/ public/ (create dest if needed). Command should complete successfully (I noticed this was more verbose in this version. The number of minified files should be the same as when using this branch.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.