I encounter the same problem when installing docker-ce version 18. I won’t use aptitude to install the old version because nvidia-docker only supports Docker version above 18!
The solution is quite easy:
sudo add-apt-repository ppa:ubuntu-sdk-team/ppa
sudo apt-get update
Then you can use command sudo apt-get install docker-ce to solve the libseccomp2 version conflict problem.
Reference: https://gist.github.com/mingrui/187f0e629007443268ce8c1414012ed5