build: Introduce "min" version of CI builder

[ParkMyCar]

Adds a "min" flavor of the CI builder image which is ~1/10th the size of the full image. Because our Bazel build is fully hermetic, it already pulls in the necessary toolchains and thus the image we use can be quite small. The "min" image can be fetched in ~30 seconds, whereas the full image takes ~2 minutes and 30 seconds. This should save about 4 minutes end-to-end in CI because both the initial mkpipeline step and the Build steps will each save 2 minutes.

Similarly most CI jobs just run other Docker images built by the initial step, so they also can use this "min" image, which should save another 2 minutes.

Motivation

Reduce CI time by ~4 minutes

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[def-]

This is incredible!