feat(manager/docker-compose): extract volume image

[danieledwardgeorgehitchcock]

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Docker, and thus docker compose allows for mounting curated images as volumes under a service declaration. A good example of where this is used is in the new PostgreSQL Extension Layers (example Compose below):

    volumes:
      - type: image
        source: ghcr.io/pglayers/pgx-pgvector:18
        target: /extensions/pgvector
        read_only: true

Renovate should pick up the image from source, and be able to re-tag / upgrade as-in the other functionality for Docker image sources.