add pytest dependency to docker container

Dockerfile

@@ -8,7 +8,7 @@ ENV HOME /root
 
 # Create the environment and install packages
 RUN conda create --name ${CONDA_DEFAULT_ENV} python=3.12.3 -y && \
-    conda install -n ${CONDA_DEFAULT_ENV} -c conda-forge fenics-dolfinx=0.8.0 mpich=4.2.1 pyvista=0.43.10 matplotlib=3.8.4 numpy=1.26.4 scipy=1.14.0 gcc=12.4.0 sphinx=7.3.7 myst-parser=4.0.0 sphinx-copybutton=0.5.2 sphinx-rtd-theme=3.0.1 -y
+    conda install -n ${CONDA_DEFAULT_ENV} -c conda-forge fenics-dolfinx=0.8.0 mpich=4.2.1 pyvista=0.43.10 matplotlib=3.8.4 numpy=1.26.4 scipy=1.14.0 gcc=12.4.0 sphinx=7.3.7 myst-parser=4.0.0 sphinx-copybutton=0.5.2 sphinx-rtd-theme=3.0.1 pytest==8.3.3 -y
 
 # Activate environment
 SHELL ["conda", "run", "-n", "fenicsx-env", "/bin/bash", "-c"]