Move CI script to main maubot repo

.gitlab-ci.yml

-image: dock.mau.dev/maubot/maubot
-
-stages:
-- build
-
-variables:
-  PYTHONPATH: /opt/maubot
-
-build:
-  stage: build
-  except:
-  - tags
-  script:
-  - python3 -m maubot.cli build -o xyz.maubot.$CI_PROJECT_NAME-$CI_COMMIT_REF_NAME-$CI_COMMIT_SHORT_SHA.mbp
-  artifacts:
-    paths:
-    - "*.mbp"
-    expire_in: 365 days
-
-build tags:
-  stage: build
-  only:
-  - tags
-  script:
-  - python3 -m maubot.cli build -o xyz.maubot.$CI_PROJECT_NAME-$CI_COMMIT_TAG.mbp
-  artifacts:
-    paths:
-    - "*.mbp"
-    expire_in: never
+include:
+- project: 'maubot/maubot'
+  file: '/.gitlab-ci-plugin.yml'