use the drone cli to check that the starlark is well-formed,
and to try to run a build.
Useful for sanity checking a pending CI commit.
Slim down the deps very slight on ubuntu systems.
This has our first end-to-end integration test that validates
smtp -> source mta -> sink mta -> maildir
and confirms that the message has the right bits inside it.
The end to end test will get refactored into more easily usable
pieces in follow-up commit(s).
Wez Furlong
drone