Update Docker and auto-build-image image versions
- Remove
-stable
from Docker image versions to make themdocker:git
anddocker:dind
(as the-stable
tags are no longer updated). - Change version of
auto-build-image
fromstable
tov1.31.0
(asstable
has been frozen since GitLab 12). - Update CHANGELOG.
Closes #37 (closed)