Use an alternate working directory for Puma

What does this merge request do and why?

This emulates what Omnibus GitLab does and should prevent issues where Rails.root has been excluded from a filename in an initializer.

Note that this works because BUNDLE_GEMFILE is specified in the environment.

Relates to gitlab-org/quality/engineering-productivity/master-broken-incidents#8341 (closed)

How to set up and validate locally

1. Run gdk reconfigure.
2. gdk restart rails-web
3. Ensure GitLab comes up.

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 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.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.