chore: add comments with links to dockerhub

Author: php-coder

.github/workflows/integration-tests-postgres.yml

@@ -26,6 +26,7 @@ jobs:
     services:
       # NB: The hostname of the service container is automatically mapped to the label name
       db:
+        # https://hub.docker.com/_/postgres
         # https://github.com/docker-library/postgres/blob/ad464b0375fc64e70e01305bf93183428a2ef0ec/11/Dockerfile
         # NOTE: it's better to have the same as in infra/docker/postgres.yml
         image: postgres:11.3

infra/docker/postgres.yml

@@ -26,6 +26,7 @@ services:
     depends_on:
       - db
   db:
+    # https://hub.docker.com/_/postgres
     # https://github.com/docker-library/postgres/blob/ad464b0375fc64e70e01305bf93183428a2ef0ec/11/Dockerfile
     # NOTE: it's better to have the same as in .github/workflows/integration-tests-postgres.yml
     image: postgres:11.3

infra/docker/prod.yml

@@ -25,6 +25,7 @@ services:
     depends_on:
       - db
   db:
+    # https://hub.docker.com/_/mysql
     # https://github.com/docker-library/mysql/blob/429047ac5e28d59d40a2ac84a189c9d25310f060/5.7/Dockerfile
     image: mysql:5.7.20
     command: --character-set-server=utf8