Improve integration testing

# Problem

1) Our current integration tests for Envbox test with the following images:
- gcr.io/coder-dev-1/sreya/enterprise-base:ubuntu
- gcr.io/coder-dev-1/sreya/hello-world
- gcr.io/coder-dev-1/sreya/ubuntu-coder

These images are manually vendored in GCR and will need to be moved elsewhere when it is discontinued (March 18, 2025).

2) We do not run integration tests with RedHat UBI9 (`registry.redhat.io/ubi9/ubi:9.5`). This is a widely used based image and is sufficiently different to warrant testing on its own.

3) We do not run integration tests for GPU support. Automated integration tests for GPU support will be difficult, but we can at least have a test that can be run manually if the required dependencies are present.

Locate the current Envbox integration tests that use the three manually vendored GCR images and review how test images are configured. Replace that dependency with an available image source, add coverage for registry.redhat.io/ubi9/ubi:9.5, and provide a manually runnable GPU-support test when its dependencies are present.

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