Taichi: Docker build is broken due to PPA removed

[jackalcooper]

Describe the bug
The ppa used in Dockerfile is no longer available. This breaks the docker build.
News: Jonath F. removed ppas

Log/Screenshots

...
Reading package lists...
W: The repository 'http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu xenial Release' does not have a Release file.
E: Failed to fetch http://ppa.launchpad.net/jonathonf/python-3.6/ubuntu/dists/xenial/main/binary-amd64/Packages  403  Forbidden
E: Some index files failed to download. They have been ignored, or old ones used instead.
The command '/bin/sh -c apt-get update && apt-get install -y     build-essential     python3.6     python3.6-dev     python3-pip     python3.6-venv     git     cmake     libtinfo-dev     wget     libx11-dev     libsm6     libxext6     libxrender-dev     zlib1g-dev     xz-utils     curl     libomp5     libomp-dev' returned a non-zero code: 100

To Reproduce
build with dockerfile

[yuanming-hu]

Oh no... The Dockerfile has not been updated for a while and I believe it will run into more problems than this one. The Jenkinsfile (https://github.com/taichi-dev/taichi/blob/master/Jenkinsfile#L119) can be used as a reference to fix it.

[jackalcooper]

glad to help. I managed to build the .so but I am not sure what to do next.
dockerfile

[yuanming-hu]

Great! You can add /app/taichi/bin to PATH and then execute

ti test

to run all the unit tests.

[jackalcooper]

Discussion moved to this PR.