`additional_contexts` with `service:` syntax not supported

[slaskis]

Docker Compose supports additional_contexts with service: syntax to reference another service's build output as a build context. This works with docker compose build (or docker bake) but fails with depot bake.

This pattern is useful for monorepos where multiple services share a common base image with pre-built dependencies. The base is built once and reused by all app images.

Minimal reproduction

services:
  base:
    image: example-base
    build:
      dockerfile_inline: |
        FROM alpine AS base
        RUN echo "base"

  app:
    image: example-app
    build:
      dockerfile_inline: |
        FROM base
        RUN echo "app"
      additional_contexts:
        base: service:base

# Works
docker bake

# Fails
depot bake

Error

Error: failed to get build context base: stat service:base: no such file or directory

Expected behavior

Depot should resolve service:base to the build output of the base service, matching docker compose build behavior.