tiltfile: make staticbuild helper (i.e. make image target assertions specific to the type of build)

[maiamcc]

Hello @landism, @nicks,

Please review the following commits I made in branch maiamcc/static-build-helper:

bb2a3ca (2019-03-13 16:11:34 -0400)
tiltfile: make staticbuild helper (i.e. make image target assertions specific to the type of build)

Code review reminders, by giving a LGTM you attest that:

• Commits are adequately tested
• Code is easy to understand and conforms to style guides
• Incomplete code is marked with TODOs
• Code is suitably instrumented with logging and metrics

[nicks]

i thought we were moving away from the StaticBuild terminology and towards the DockerBuild terminology? and this seems to move us in the wrong direction?

[maiamcc]

this PR isn't about the terminology as much as making the helper explicitly assert what kind of build it is (as opposed to just checking the fields common to staticBuild/fastBuild). I'm super down to rename this later, but i'd rather not do a big rename in the codebase before we've even phased out fastBuild in its current form.

[nicks]

ah, i'm confused, this pr does a big rename from db -> sb? why not just leave it as db?

[maiamcc]

...oh. because we're asserting on the staticBuild struct, not the call to docker_build that created it.

[maiamcc]

...i'm rolling back most of this rename as part of a PR getting ready for LiveUpdate and i'm full of chagrin 😅