Migrate images/gce-proxy to standalone repository

[danisla]

The GCE proxy is used by the vdi-vm example to create a localhost proxy that uses the GCE VM service account credentials to transparently authenticate to the pod broker.

This should be moved to a top-level selkies-project repo so it can be maintained separately and should remove most/all of the dependabot warnings.

TODO:

• Copy images/gce-proxy to new repo in selkies-project
• Remove cloud builder
• Remove any other references in this repo.
• Create GH workflow to automate image building.
• Update selkies-examples/vdi-vm if necessary.