CI: define pull policy for image in gitlab-ci.yml

Description

Currently, image pull policy is defined on the GitLab runner level. gitlab-runner!115 (merged)

Proposal

It should be possible to define it in gitlab-ci.yml. Example:

build:
  image:
    name: $CI_REGISTRY/dreamhost/kubectl-helm-docker
    pull: always # available: always, if-not-present, never
  script:
  - ...

The desired pull policy is context-dependent. We need to use the very latest images in some cases, and we can use a cached version in other cases. Since it's context dependent, it perfectly belongs to .gitlab-ci.yml. 😃

Links / references

gitlab-runner!115 (merged)

@markpundsack @ayufan @grzesiek @markglenfletcher