Allow a global prebuild repo

[mrsimonemms]

Is your feature request related to a problem? to Currently, the rebuild repo is tied to a workspace.

It would be useful to be able to configure a global rebuild repo to retrieve images. As the images are stored by tag, this should be a fairly straightforward change. I do store my prebuilds in a single repo (https://github.com/users/mrsimonemms/packages/container/package/devpod-prebuilds), so this should be fairly straightforward.

Which solution do you suggest?
A setting under the Settings -> General (and something similar for the CLI) to configure the repo (eg, ghcr.io/mrsimonemms/devpod-prebuilds). This does already work if the prebuild is configured for a workspace.

For bonus points, it would handy to be able to also save any images built locally in this repo too. It will probably require additional authentication parameters, which may be out of scope (or too complex) but would be a game-changer in improving the workspace start speed.

Which alternative solutions exist?

Additional context