Put pacxx++ into $PATH in the generated images

Closes: #28

pacxx-perftest-2.6/Dockerfile

@@ -18,6 +18,7 @@ RUN set -ex;                                            \
     rm -rf /var/lib/apt/lists/*
 
 COPY --chown=root:root opt/pacxx /opt/pacxx
+ENV PATH=/opt/pacxx/bin:$PATH
 
 USER duneci
 

pacxx/Dockerfile

@@ -2,3 +2,4 @@ ARG TAG
 FROM registry.dune-project.org/joe/pacxx-docker:$TAG
 
 COPY --chown=root:root opt/pacxx /opt/pacxx
+ENV PATH=/opt/pacxx/bin:$PATH