fix: Add libgsl-dev for pyccl build

cs_util 0.1.9 requires pyccl, which needs GSL. Pre-installing
libgsl-dev avoids pyccl trying to download GSL from FTP.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cailmdaley

Dockerfile

@@ -27,6 +27,7 @@ RUN apt-get update -y --quiet --fix-missing && \
     libfftw3-bin \
     libfftw3-dev \
     libgl1-mesa-glx \
+    libgsl-dev \
     libtool \
     libtool-bin \
     libtool-doc \