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.

Finde die aktuellen Envbox-Integrationstests, die die drei manuell vendorten GCR-Images verwenden, und prüfe, wie Test-Images konfiguriert werden. Ersetze diese Abhängigkeit durch eine verfügbare Image-Quelle, füge Abdeckung für registry.redhat.io/ubi9/ubi:9.5 hinzu und stelle einen manuell ausführbaren GPU-Support-Test bereit, wenn seine Abhängigkeiten vorhanden sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.