fix: restrict libvips to trusted image loaders (#15254)

## Description

Adds `VIPS_BLOCK_UNTRUSTED=1` to `.env.example`. This tells libvips to
only use its trusted, well-tested loaders when Active Storage generates
image variants, hardening image processing against untrusted uploads.
The setting requires libvips >= 8.13 and is silently ignored on older
versions.

Related to https://linear.app/chatwoot/issue/INF-92

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

Verified locally that thumbnail/variant generation for the common raster
formats (JPEG, PNG, GIF, WebP, TIFF, HEIC) is unaffected with the flag
enabled.

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code

docker/Dockerfile

@@ -130,6 +130,9 @@ RUN apk update && apk add --no-cache \
   vips \
   && gem install bundler -v "$BUNDLER_VERSION"
 
+# Restrict libvips to its trusted image loaders when generating variants
+ENV VIPS_BLOCK_UNTRUSTED=1
+
 COPY --from=node /usr/local/bin/node /usr/local/bin/
 COPY --from=node /usr/local/lib/node_modules /usr/local/lib/node_modules