Enable SSH access for GDK image in Remote Development workspace

Issue: Enable SSH access for GDK image in Remote Devel... (#1987)

What does this merge request do and why?

Adds SSH access to GDK workspaces docker file as described in Enable SSH access for GDK image in Remote Devel... (#1987)

See https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/doc/ssh.md#updating-your-runtime-images-to-allow-ssh-connections for current instructions to add SSH support to a workspace docker image.

How to set up and validate locally

• From GDK dir, run ./support/docker-build support/gitlab-remote-development workspace to build the image
• Verify there are no errors in output
• Use the resulting image to create and run a remote development workspace for the GDK, according to the instructions in https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/gitlab-remote-development.md
• Successfully connect to the image via SSH (TODO: add detailed instructions)

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.