Setup CD for dpkg updates

[cmrust]

Create CircleCI job to continuously push and tag new builds with the Linux host agent version number.

[cheempz]

Some background @alehandru -- we need a way to trigger a rebuild of this host agent docker image to ensure the latest host agent is baked in. This issue is going to set up a CircleCI job to just automatically build this every two weeks, but would be even better if we can make it happen only when a new Linux Host Agent is released.

Seems CircleCI has an API to trigger new builds: https://circleci.com/docs/api/v1-reference/#new-build. Would it be doable to add this trigger to our ship_agent script, when promoting a package to the prod repo?