heptapod-ci: allow testing with docker image other than :latest

The project have a default config of latest for this variable.

Differential Revision: https://phab.mercurial-scm.org/D9792

contrib/heptapod-ci.yml

@@ -2,7 +2,7 b' stages:'
   - tests
   - phabricator
 
-image: registry.heptapod.net/mercurial/ci-images/mercurial-core
+image: registry.heptapod.net/mercurial/ci-images/mercurial-core:$HG_CI_IMAGE_TAG
 
 variables:
     PYTHON: python