For the person sending the MR to update an image tag, the CI would build
a new image in the user repository. However, next time this user does
a CI pipeline, the image used will be the one from this user, not the
upstream one. This can lead to some issues:
- we have to pull a different image for that user
- the user image is different, and might not be correct, hiding some
  issues (for example if 2 MR set the same tag).

Ensure the used image is always the upstream one, by comparing the
various sha (digest and layers).

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>