Cleanup dockerfile

I believe https://eicweb.phy.anl.gov/containers/eic_container/-/blob/master/containers/jug/dev.Dockerfile#L34

 && mkdir -p $SPACK_ROOT/opt/spack                                      \

works accidentally and should be

 && mkdir -p $SPACK_ROOT                                      \

because of https://eicweb.phy.anl.gov/containers/eic_container/-/blob/master/containers/jug/dev.Dockerfile#L23