Refactor docker implementation to separate container build in CI

[ircmaxell]

Currently there are 2 containers that are compiled every commit: 16.04 and 16.04-dev. The 16.04 container extends the 16.04-dev and adds the compiler (where the 16.04-dev is intended to run on your own checkout to aid execution).

The 16.04 container should be re-built on every commit, since it includes code.

However, the 16.04-dev container can be rebuilt far less frequently. Perhaps once per week, unless there's a change in the /Docker folder.

Circle.yml should be refactored to only run make docker-build-clean once per week, or if /Docker changes.

And on cases where we don't run make docker-build-clean, run docker build --no-cache -t ircmaxell/php-compiler:16.04 -f Docker/ubuntu-16.04/Dockerfile . instead.

[driusan]

There's some other refactoring that the config can use too:

1. Make it use the docker environment instead of the machine environment (after the above is done)
2. Run the phan/phpunit tests in parallel instead of in sequence