Update the Dockerfile for stable, Ubuntu 16.04-based builds

[sethfowler]

This PR makes several changes to the Dockerfile to bring it in sync with changes made in upstream repos:

• It's now based on Ubuntu 16.04.
• It's built from the stable tag of behavioral-model.
• It supports different image variants for normal builds and testing. This lets downstream repos which don't need to run the tests use a smaller version of the image. (To support this, we need to install p4c globally in the Dockerfile, so we now run make install.) The approach is the same one we used in the behavioral-model repo.
• Just as in upstream repos, we're now excluding unneeded files via a .dockerignore file.

[sethfowler]

By the way, the tests will fail until I merge the PRs in third-party and behavioral-model. Once this code gets reviewed, I'll merge them all at the same time.

[sethfowler]

So p4c currently depends on BMV2 features which aren't in the BMV2 stable branch. At the moment, unfortunately, we'll have to continue depending on behavioral-model:latest. When another release of BMV2 is cut, we can cut a stable release of p4c at that point.