react-express-mongodb: rename server to backend

The goal here is to keep the same structure than the other examples.
Also, considering that it has a "frontend" folder, the equivalent for
the server should be "backend".

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-express-mongodb/backend/README.md

@@ -0,0 +1,41 @@
+#### Snippet of backend(Node.js)`DockerFile`
+
+You will find this `DockerFile` file in the root directory of the project.
+
+```bash
+FROM node:13.13.0-stretch-slim
+#Argument that is passed from docer-compose.yaml file
+ARG NODE_PORT
+#Echo the argument to check passed argument loaded here correctly
+RUN echo "Argument port is : $NODE_PORT"
+# Create app directory
+WORKDIR /usr/src/app
+#COPY . .
+COPY . .
+# Install app dependencies
+# A wildcard is used to ensure both package.json AND package-lock.json are copied
+# where available (npm@5+)
+RUN npm install
+#In my case my app binds to port NODE_PORT so you'll use the EXPOSE instruction to have it mapped by the docker daemon:
+EXPOSE ${NODE_PORT}
+CMD npm run dev
+```
+
+##### Explanation of backend(Node.js) `DockerFile`
+
+- The first line tells Docker to use another Node image from the [DockerHub](https://hub.docker.com/). We’re using the official Docker image for Node.js and it’s version 10 image.
+
+- On second line we declare argument `NODE_PORT` which we will pass it from `docker-compose`.
+
+- On third line we log to check argument is successfully read 
+
+- On fourth line we sets a working directory from where the app code will live inside the Docker container.
+
+- On fifth line, we are copying/bundling our code working directory into container working directory on line three.
+
+- On line seven, we run npm install for dependencies in container on line four.
+
+- On Line eight, we setup the port, that Docker will expose when the container is running. In our case it is the port which we define inside `.env` file, read it from `docker-compose` then passed as a argument to the (backend)`DockerFile`.
+
+- And in last, we tell docker to execute our app inside the container by using node to run `npm run dev. It is the command which I registered in __package.json__ in script section.
+###### :clipboard: `Note: For development purpose I used __nodemon__ , If you need to deploy at production you should change CMD from __npm run dev__ to __npm start__.`