[Dockerfile] Upgrade debian base image to version 10

Debian8 is too old to build LLVM project, the version
of GCC, CMake and python are lower than the requirements:
https://llvm.org/docs/GettingStarted.html#software

Debian10 is the earliest release that has software
packages that meet the above requirements.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D120826