Support multiple image builds in custom builders

[cmellard]

Describe the Feature You Want

When running builds/runs with Bazel it is efficient to define targets that build and push multiple images in one go rather than having to orchestrate multiple Bazel runs from Tilt and queuing up a large series of these - it's almost always more efficient to use multirun in Bazel to push multiple oci images to a remote or load them locally. Even if we let Tilt handle the pushing which does work concurrently we still have a sequential problem with Bazel run and oci_load loading these images in to the local docker registry. Not entirely sure how the contract would look for custom_build but I imagine some list of image refs to build that we can pipe to bazel rathe than a single ref.

Current Behavior

We define a custom_build for each image and Bazel queues up many bazel run and we wait a long time until it has finished building and pushing all of those.

Why Do You Want This?

Tilt is unusable when using Bazel as a custom builder when you have a high number of images that share cross cutting libraries and need rebuilding at the same time. The cycle time is very high.

[nicks]

it's been a while since i worked on Bazel stuff 🧓 but shouldn't rebuilding an artifact that's already been built be pretty fast? like <200ms? like, the whole point of the Bazel graph was to avoid having to do your own build graph on top of bazel.

or is this an artifact of how the oci image bazel rules work?