🚚(docker) make images naming consistent

It was quite confusing having development, production and
frontend images' names in the same Docker file. New comers
to the project would have some difficuluties when
differentiating frontend from backend images.

Try to make these naming more explicit and consistent.
Thanks @rouja for your recommendation.

docker-compose.yml

@@ -19,11 +19,11 @@ services:
   app-dev:
     build:
       context: .
-      target: development
+      target: backend-development
       args:
         DOCKER_USER: ${DOCKER_USER:-1000}
     user: ${DOCKER_USER:-1000}
-    image: people:development
+    image: people:backend-development
     environment:
       - PYLINTHOME=/app/.pylint.d
       - DJANGO_CONFIGURATION=Development
@@ -43,7 +43,7 @@ services:
   
   celery-dev:
     user: ${DOCKER_USER:-1000}
-    image: people:development
+    image: people:backend-development
     command: ["celery", "-A", "people.celery_app", "worker", "-l", "DEBUG"]
     environment:
       - DJANGO_CONFIGURATION=Development
@@ -60,11 +60,11 @@ services:
   app:
     build:
       context: .
-      target: production
+      target: backend-production
       args:
         DOCKER_USER: ${DOCKER_USER:-1000}
     user: ${DOCKER_USER:-1000}
-    image: people:production
+    image: people:backend-production
     environment:
       - DJANGO_CONFIGURATION=Demo
     env_file:
@@ -78,7 +78,7 @@ services:
 
   celery:
     user: ${DOCKER_USER:-1000}
-    image: people:production
+    image: people:backend-production
     command: ["celery", "-A", "people.celery_app", "worker", "-l", "INFO"]
     environment:
       - DJANGO_CONFIGURATION=Demo