Increase inotify watches in gitpod-docker-image job

What does this merge request do and why?

gitlab!105145 (merged) reintroduced the use of the listen gem to watch for code changes. We need to make sure the sysctl setting is high enough to work with GitLab Rails.

Relates to #1625 (closed)

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.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.