try to add Docker image building in CI

.gitlab-ci.yml

 image: python:3.9  # Use a Python Docker image
 
 stages:  # Define stages in the pipeline
+  - prepare
   - build
   - deploy
 
+prepare:
+  image: docker:20.10.16
+  stage: build
+  services:
+    - docker:20.10.16-dind
+  before_script:
+    - echo "$CI_REGISTRY_PASSWORD" | docker login $CI_REGISTRY -u $CI_REGISTRY_USER --password-stdin
+  script:
+    - docker pull $CI_REGISTRY_IMAGE:latest || true
+    - docker build
+      --cache-from $CI_REGISTRY_IMAGE:latest
+      --tag $CI_REGISTRY_IMAGE:latest .
+    - docker push $CI_REGISTRY_IMAGE:latest
+
 # Job to build documentation
 build-docs:
   stage: build
@@ -23,4 +38,4 @@ pages:
     paths:
       - public  # Files in the "public" folder will be deployed to GitLab Pages
   only:
-    - main  # Only deploy if the changes are in the default branch
\ No newline at end of file
+    - main  # Only deploy if the changes are in the default branch