Resolve GitLab startup issue in verify gitpod-docker-image job

What does this merge request do and why?

This MR updates the gitpod-command script, which reflects the commands that run during the launching of Gitpod workspaces, as described in the below description (quoting it from gitlab!122005 (merged)):

This MR runs the gdk update within the Gitpod workspace. Previously, we only ran gdk reconfigure, which worked because it included duplicated commands from gdk update. But, now that there are no more duplicated commands between gdk update and gdk reconfigure, it is necessary to run this command in the Gitpod workspace.

Also, this MR updates the health check of the GitLab in GDK gitpod docker image by checking /users/sign_in instead of /-/readiness/ due to this issue: #1839 (closed).

Related to #1836 (closed)

How to set up and validate locally

An example successful verify gitpod-docker-image job: https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/4369796458

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.