feat: specify base images to refresh on deploy

[bomoko]

Currently, if one chooses to implement a base image solution -- that is, most of one's code being bundled in an upstream image shared by multiple projects/environments -- there is an issue with caching.

If the base image, say, container.reg/my/baseimage:latest, or indeed, anything that isn't injected explicitly into the build, the image is cached by the docker-host, and if you make changes to the upstream image, but container.reg/my/baseimage:latest is currently in the docker-host's cache, it's not going to pull the new upstream image.

This small change allows users to explicitly mark their services as using a base image, and specifying what the name of that image is via an optional label lagoon.base.image.

This leads to, for example, service definitions that look like so:

  cli: # cli container, will be used for executing composer and any local commands (drush, drupal, etc.)
    build:
      context: .
      dockerfile: lagoon/cli.dockerfile
    image: &cli-image ${COMPOSE_PROJECT_NAME:-drupal-base}-cli # this image will be reused as `CLI_IMAGE` in subsequent Docker builds
    labels:
      # Lagoon Labels
      lagoon.type: cli-persistent
      lagoon.persistent.name: nginx
      lagoon.persistent: /app/web/sites/default/files/ 
      lagoon.base.image: container.reg/my/baseimage:latest
      lando.type: php-cli-drupal
 <snip/>

On finding the label, the build-deploy-tool with attempt to pull the upstream image, which it will do if there have been updates.

[shreddedbacon]

This solves the problem neatly, but also in a way that allows for expansion in the future if necessary.