Remove gitlab ci deprecated keywords

According to: https://docs.gitlab.com/ee/ci/yaml/#deprecated-keywords

.gitlab-ci.yml

-image:
-  name: docker:20.10.17
+default:
+  image: $CI_REGISTRY_IMAGE:latest
 
 variables:
   CAMPUS_IP: 138.26.48.47
@@ -40,6 +40,7 @@ workflow:
     - if: $CI_PIPELINE_SOURCE == 'schedule'
 
 build_docker_image:
+  image: docker:20.10.17
   stage: pre-build
   services:
     - docker:20.10.16-dind
@@ -68,7 +69,6 @@ build_docker_image:
       allow_failure: true
 
 build_image:
-  image: $CI_REGISTRY_IMAGE:latest
   stage: build
   environment:
     name: knightly
@@ -139,7 +139,6 @@ build_image:
       dotenv: image.env
 
 test:
-  image: $CI_REGISTRY_IMAGE:latest
   stage: test
   environment:
     name: knightly
@@ -193,7 +192,6 @@ test:
       dotenv: instance.env
 
 deploy_review:
-  image: $CI_REGISTRY_IMAGE:latest
   stage: deploy
   script:
     - echo "Deploy Review App"
@@ -208,7 +206,6 @@ deploy_review:
     - if: $CI_MERGE_REQUEST_ID
 
 stop_review:
-  image: $CI_REGISTRY_IMAGE:latest
   stage: deploy
   script:
     - openstack server delete $NEW_INSTANCE_ID
@@ -224,7 +221,6 @@ stop_review:
       when: manual
 
 deploy_knightly:
-  image: $CI_REGISTRY_IMAGE:latest
   stage: deploy
   environment:
     name: knightly