Added docker pull policy

Kamil Trzcińskirequested to merge
pull-policy into master

This removes the image_ttl from [runners.docker] and replaces it with pull_policy.

The values:

  • always - pull image every time default,
  • if-not-present - pull image for the first time only,
  • never - don't pull image, fail if the image is not present,

@tmaczukin Please review

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