Add imlib2 to Docker build deps for Lens image viewer

Lens links against libimlib2 at compile time. The package was already
in the ISO runtime list but missing from the builder image.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/docker/Dockerfile

@@ -20,6 +20,7 @@ RUN pacman -Syu --noconfirm && \
         libxft \
         fontconfig \
         freetype2 \
+        imlib2 \
         imagemagick \
         fakeroot \
         git \