Update Customization documentation on how to download gated models.

[Andergraw]

The docs about customization are awesome. It would be amazing to have more info on how to download gated
models (from Hugging Face or CivitAI) without exposing the keys.
With Hugging Face, probably using theur CLI and with CivitAI, just storing the secret securely and adding it as an env variable or something.
Thanks!

[TimPietruskyRunPod]

Great call. Documenting this gap to be addressed:

Hugging Face (gated models like FLUX.1-dev)

The existing pattern works — HUGGINGFACE_ACCESS_TOKEN is passed as a build-arg through to the download step in the multi-stage Dockerfile, and the token doesn't end up in the final image because the COPY --from=downloader only brings the model files forward. See Dockerfile:119-142 for the exact pattern used by the official builds. I wrote up a derived-image example in #149 — I'll lift that into docs/customization.md as part of this issue.

CivitAI

Same pattern works for CivitAI's API token — pass CIVITAI_TOKEN as a build arg and use it in a wget --header="Authorization: Bearer ...". The download URL pattern is https://civitai.com/api/download/models/<model-version-id>?token=<TOKEN>.

Network volume alternative

For things you don't want to bake into images at all (or that change frequently), pre-load to a network volume from a pod with credentials, then attach the volume to your serverless endpoint. The serverless worker never sees the credentials. PR #193 (open) adds guidance on this.

Tagging as a docs improvement — happy to take a PR that adds a "Downloading gated models" section to docs/customization.md covering all three. Leaving open until that lands.