Run composer install inside the container to allow for platform constraints, path-based config

Problem/Motivation

Currently we have a composer install step which runs during the assemble_codebase phase.

This is fine for some sanity checks and also other requirements, but it also brings some problems.

It could be that the platform where we perform the code assemble breaks some composer platform requirements which are different from those in the container, such as different PHP versions or available extensions.

We also have some core changes coming where Composer hooks are used to make path-based configurations. For this, we need the container paths rather than the host paths. #2744463: Add phpcs, coder 8.2.8 as --dev requirements for drupal/core

Proposed resolution

Add a composer install plugin which can occur after the containers are available.

Place this plugin in the build definition as the first step during validate_codebase:.

Remaining tasks

User interface changes

API changes

Data model changes