Move to Slim Image as Default

[0xThresh]

Open WebUI now has slim images as of v0.6.26:
Added new slim Docker image option via "USE_SLIM" build argument that excludes embedded AI models and Ollama installation, reducing image size by approximately 1GB. This variant enables faster image pulls and deployments for environments where AI models are managed externally, particularly beneficial for auto-scaling clusters and distributed deployments.

This seems like it should become the default for Open WebUI's Helm charts with a new value (such as use_slim: true) that can still allow users to pull the full image. This would lead to quicker pull times/ deployments, but may cause inconvenience to not have the embedded AI models.