fix: strip additional_contexts and resolve relative build contexts

[AaronFeledy]

Problem

Two related build issues with the Lagoon plugin:

1. additional_contexts with service: references fail — Upstream lagoon-examples/drupal-base added additional_contexts in Aug 2025 to enforce BuildKit build ordering. Lando splits services into separate compose files, so Docker can't resolve service:cli across files and treats it as a filesystem path (~/.lando/compose/drupalbase/service:cli), causing all leia-tests to fail.

2. build.context always forced to project root — _lagoon.js rewrites build.context to path.join(options.root) regardless of the original value, breaking monorepo setups where services use non-root contexts (e.g. context: frontend).

Fix

• Strip build.additional_contexts in lib/config.js during service parsing. The CLI_IMAGE build arg already handles cross-service image references (ARG CLI_IMAGE → FROM ${CLI_IMAGE} AS cli), making additional_contexts redundant for local dev.

• Use path.resolve instead of path.join in _lagoon.js so relative build contexts resolve correctly against the project root. context: . still resolves to root (no regression), but context: frontend now correctly resolves to <root>/frontend.

Testing

• Unit tests pass
• The leia-test failures in CI (drupal9-base, all-services, proxy-tests) should be resolved by this fix since the upstream drupal-base example now uses additional_contexts

Fixes #7
Fixes #53

---

Note

Medium Risk
Touches Lagoon service build configuration, which can affect how images are built and may break projects relying on current (incorrect) context rewriting or on additional_contexts behavior. Scope is limited to local compose/service parsing and build context normalization (no auth/data handling).

Overview
Fixes Lagoon build compatibility in two places.

builders/_lagoon.js now resolves build.context relative to the project root (instead of forcing root), enabling non-root build contexts in monorepos. lib/config.js strips build.additional_contexts during service parsing to avoid Docker service: context references that can’t resolve when Lando splits services into separate compose files.

Updates the Drupal 9 base example README with verification steps to confirm images build via the shared CLI image and that generated compose files do not contain additional_contexts.

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[tobybellwood]

Not sure if you want a formal review from us, but this makes sense - when users first encountered this with Lando, we told them to remove the additional_contexts, so I can't foresee it causing too many issues. Obviously, any newcomers can use the templates straight OOTB now.