docker: Update Dockerfile
docker: Update Dockerfile

 - Remove python2 stuff
 - Build python3 bindings and install via pip
 - More cleaning / smaller size image

soonho-tri/dreal4,dreal/dreal4,dreal/dreal4,dreal/dreal4,soonho-tri/dreal4,soonho-tri/dreal4,dreal/dreal4,soonho-tri/dreal4