Add a HEALTHCHECK to the images

When helping people on StackOverflow, I often see people having problems with Docker Compose and applications that try to connect to the database immediately on startup. When using a simple docker compose 'depends_on' directive, compose only waits for the database to be started before starting the application. The application then fails because the database isn't ready to accept connections yet.

The solution is to add a healthcheck to the database container in docker compose and have the application wait for the database to enter a healthy state.

It would be nice if the healthcheck was included in the Postgres image by default. A simple HEALTHCHECK ["pg_isready"] should be enough.

Empieza por localizar las definiciones de imágenes de Docker para las imágenes de Postgres y comprobar cómo se crean sus variantes. Añade de forma coherente el healthcheck solicitado y verifica después que Docker Compose pueda esperar a que la base de datos indique un estado saludable antes de iniciar la aplicación.