Use absolute path for constraints file in Dockerfile

Author: connorjward

docker/Dockerfile.vanilla

@@ -146,8 +146,8 @@ ENV CFLAGS="-O3 -mtune=generic" CPPFLAGS="-O3 -mtune=generic"
 #     before UFL otherwise `pip install fiat` will reinstall pypi ufl.
 
 # Fix for petsc4py+slepc4py build to be removed after #4873 is resolved.
-RUN echo 'setuptools<81' > constraints.txt
-ENV PIP_CONSTRAINT=constraints.txt
+RUN echo 'setuptools<81' > /opt/constraints.txt
+ENV PIP_CONSTRAINT=/opt/constraints.txt
 
 RUN git clone --branch ${BRANCH} \
       https://github.com/firedrakeproject/firedrake.git /opt/firedrake \