Pull images used in volumes with type=image

[Divyanshupandey007]

Description

When running docker compose pull, images referenced in volume mounts with
type=image were not being pulled. Only the main service images (service.Image)
were considered.
The pullRequiredImages() function (used by docker compose up) already handled
this case by iterating through service volumes and creating fake ServiceConfig
entries for volume images. However, the pull() function (used by docker compose pull)
did not have this logic.
This adds the same volume image discovery to the pull() function so that
docker compose pull also pulls images referenced in type=image volume mounts.

Changes

• pkg/compose/pull.go: Added a loop in pull() that iterates each service's
  volumes, checks for type=image, deduplicates against imagesBeingPulled, and
  queues them for pulling via pullServiceImage().

How to test

services:
  nginx:
    image: "nginx:alpine"
    volumes:
      - type: image
        source: mattfly/obsidian:latest
        target: /srv/http/ObsidianIRC
        image:
          subpath: usr/share/nginx/html/

Fixes #13809