in docker-compose.yml, what does this mean?ports: – “${BACKEND_PORT:-80}:80”

In a Docker Compose file (docker-compose.yml), the ports section is used to expose network ports for services defined in the file. It allows containers to listen on specified ports and make them accessible from outside the Docker environment.

In the given example, the ports section is using the following format:

ports:
  - "${BACKEND_PORT:-80}:80"

This line maps the host machine’s port to the container’s port. Let’s break it down:

• ${BACKEND_PORT:-80}: This is a variable substitution that allows flexibility in specifying the host machine’s port. It uses the value of the environment variable BACKEND_PORT if it is defined, or falls back to the default value of 80 if the variable is not set. It means that you can override the default port by setting the BACKEND_PORT environment variable.
• 80:80: This represents the port mapping in the format <host_port>:<container_port>. In this case, it maps port 80 of the container to the same port 80 of the host machine. So, any traffic that arrives at the host’s port 80 will be forwarded to the container’s port 80.

Overall, this configuration allows you to access the backend service running inside the container via the host machine’s port 80 (or the custom port specified by the BACKEND_PORT environment variable). For example, if the host machine’s IP address is 192.168.0.1, you can access the backend service by visiting http://192.168.0.1 on your web browser.