Docker: OpenMPI Root

Fix the root work-around (`export -f mpi...`) and include
mpiexec as well.

Author: ax3l

share/picongpu/dockerfiles/ubuntu-1604/Dockerfile

@@ -61,7 +61,12 @@ RUN        /bin/echo -e "source $SPACK_ROOT/share/spack/setup-env.sh\n" \
                         "spack load $PIC_PACKAGE\n" \
                         'if [ $(id -u) -eq 0 ]; then\n' \
                         '   function mpirun { $(which mpirun) --allow-run-as-root $@; }\n' \
-                        'fi' \
+                        'fi\n' \
+                        'export -f mpirun\n' \
+                        'if [ $(id -u) -eq 0 ]; then\n' \
+                        '   function mpiexec { $(which mpiexec) --allow-run-as-root $@; }\n' \
+                        'fi\n' \
+                        'export -f mpiexec\n' \
                > /etc/profile.d/picongpu.sh
 
 # build example for out-of-the-box usage: LWFA