(Nice to have) Implement pushing of intermediate images containing the `envbuilder` binary for later resumption

The purpose of this feature is to improve workspace build/startup speed even when some image layers have changed.

As shown in a PoC: https://github.com/coder/envbuilder/issues/185#issuecomment-2143411074, it's possible to create and push intermediate images during the build process. However, doing so breaks the final image.

Investigate if this is still an issue and/or if we can work around it and create the intermediate images without affecting the final image. (For instance, our refactor of creating "reproducible" images in Kaniko might have improved the situation.)

A tip: Look into cloning and restoring the build args before/after creating the image (`(*stageBuilder).args.Clone()`).

Start with the PoC linked in the issue and inspect the Kaniko reproducible-image refactor. Investigate (*stageBuilder).args.Clone() and how build arguments are restored around intermediate image creation. Done means determining whether intermediate images can be pushed for later resumption without breaking the final image, with a workaround if possible.

Scritto dal modello di indicizzazione a partire dal testo della issue.