docs: add comments in dockerfile

backend/Dockerfile

@@ -1,7 +1,21 @@
+# Use the official Node.js 21 image from Docker Hub as the base image
 FROM node:21
+
+# Set the working directory in the Docker container to /app
 WORKDIR /app
+
+# Copy package.json and package-lock.json (if available) to the working directory
 COPY package*.json ./
+
+# Install the dependencies defined in package.json
 RUN npm install
+
+# Copy the rest of the application code to the working directory
 COPY . .
+
+# Expose port 4000 in the Docker container. This is the port that the application will use.
 EXPOSE 4000
+
+# Define the command that should be executed when the Docker container starts.
+# In this case, "npm run start" will be executed.
 CMD ["npm", "run", "start"]