[Docker] mpiexec not set

Issue:

#18 84.53 This version of Spack (openmpi ~legacylaunchers schedulers=slurm) 
#18 84.53 is installed without the mpiexec/mpirun commands to prevent 
#18 84.53 unintended performance issues. See https://github.com/spack/spack/pull/10340 
#18 84.53 for more details.
#18 84.53 If you understand the potential consequences of a misconfigured mpirun, you can
#18 84.53 use spack to install 'openmpi+legacylaunchers' to restore the executables.
#18 84.53 Otherwise, use srun to launch your MPI executables.

Fix:

export MPIEXEC=orterun