Allow CI jobs using the base image to fail on a base build

.gitlab-ci.yml

-image: registry.gitlab.com/arkindex/backend/base:django-3.1.1
 stages:
   - test
   - build
   - deploy
 
+# GitLab provides a template to ensure pipelines run only for branches and tags, not for merge requests
+# This prevents duplicate pipelines in merge requests.
+# See https://docs.gitlab.com/ee/ci/troubleshooting.html#job-may-allow-multiple-pipelines-to-run-for-a-single-action
+include:
+  - template: 'Workflows/Branch-Pipelines.gitlab-ci.yml'
+
 # For jobs that run backend scripts directly
 .backend-setup:
+  image: registry.gitlab.com/arkindex/backend/base:django-3.1.1
   cache:
     paths:
       - .cache/pip
@@ -19,6 +25,13 @@ stages:
     - pip install -r tests-requirements.txt codecov
     - "echo 'database: {host: postgres, port: 5432}' > $CONFIG_PATH"
 
+  # Those jobs require the base image; they might fail if the image is not up to date.
+  # Allow them to fail when building a new base image, to prevent them from blocking a new base image build
+  rules:
+    - if: '$CI_COMMIT_TAG && $CI_COMMIT_TAG =~ /^base-.*/'
+      allow_failure: true
+    - when: on_success
+
   variables:
     # For the postgres image
     POSTGRES_DB: arkindex_dev
@@ -154,6 +167,7 @@ backend-build-binary:
 
 backend-static-deploy:
   stage: deploy
+  image: python:3-slim
 
   # Run this on any version tag except base images
   rules:
@@ -174,6 +188,7 @@ backend-static-deploy:
 
 backend-openapi-deploy:
   stage: deploy
+  image: python:3-slim
 
   # Run on master updates only
   only:
@@ -190,8 +205,8 @@ backend-openapi-deploy:
     - aws s3 cp output/schema.yml s3://teklia-assets-release/arkindex/openapi.yml
 
 sentry-release:
-  image: getsentry/sentry-cli
   stage: deploy
+  image: getsentry/sentry-cli
 
   rules:
     - if: '$CI_COMMIT_TAG && $CI_COMMIT_TAG !~ /^base-.*/'