Add support for docker caching options

Add support for docker buildx build --cache-from and --cache-to options. Together with GitLab CI/CD caching, it can considerably improve the performance of container image builds.

If the CACHE_FROM environment variable is set, the --cache-from will be added to docker buildx build command with the value of the CACHE_FROM variable. Similarly, if the CACHE_TO environment variable is set, the --cache-to will be added to docker buildx build command with the value of the CACHE_TO variable.

Example usage:

build_app:
  cache:
    key: my-app
    paths:
      - .cache/my-app/
  extends:
    - .build_docker
  variables:
    PUSH_IMAGE: "true"
    REGISTRY_IMAGE_PATH : "registry.cern.ch/my-project/my-repo:${CI_COMMIT_SHA}"
    CONTEXT_DIR: ${CI_PROJECT_DIR}
    DOCKER_FILE_NAME: Dockerfile
    CACHE_FROM: "type=local,src=${CI_PROJECT_DIR}/.cache/my-app"
    CACHE_TO: "type=local,dest=${CI_PROJECT_DIR}/.cache/my-app"