Add composer to Dockerfile from official composer image

[shaundrong]

Copy the latest 2.9.x build of composer to the Docker image.

[kurowski]

I'm curious if/why you think we need to add this? AFAICT it's a no-op:

$ docker run --rm ghcr.io/uceap/devcontainer-drupal:main /usr/local/bin/composer --version
Composer version 2.9.8 2026-05-13 09:28:38
PHP version 8.3.31 (/usr/local/bin/php)
Run the "diagnose" command to get more detailed diagnostics output.

$ docker run --rm ghcr.io/uceap/devcontainer-drupal:pr-47 /usr/local/bin/composer --version
Composer version 2.9.8 2026-05-13 09:28:38
PHP version 8.3.31 (/usr/local/bin/php)
Run the "diagnose" command to get more detailed diagnostics output.