How to build a custom Docker image?

That's an excellent question, and it's a fundamental part of working with Docker!

You already provided a great example of what goes into it with your Dockerfile snippet. To build a custom Docker image, you generally follow these steps:

Create a Dockerfile:
This is a text file named Dockerfile (no file extension) that contains a series of instructions on how to build your image. Each instruction creates a new layer on top of the previous one.

Your example Dockerfile is perfect for this:
```
FROM nginx
RUN echo "Hello from custom layer" > /usr/share/nginx/html/hello.html
```
- FROM nginx: This is always the first instruction and specifies the base image your custom image will be built upon. Here, it uses the official Nginx image.
- RUN: This instruction executes any commands in a new layer on top of the current image and commits the results. In your case, it creates a hello.html file inside the Nginx web root.
Save the Dockerfile:
Save this Dockerfile in an empty directory. It's a good practice to keep your Dockerfile and any related application files (like your hello.html if it were a separate file) in a dedicated project directory.
Use the docker build command:
Open your terminal, navigate to the directory where you saved your Dockerfile, and then run the docker build command:
```
docker build -t my-custom-nginx .
```
- docker build: This is the command that Docker uses to build images from a Dockerfile.
- -t my-custom-nginx: The -t (or --tag) flag allows you to give your new image a human-readable name and optionally a tag (version). Here, I've named it my-custom-nginx. If you wanted to specify a version, you could do my-custom-nginx:v1.0.
- .: This is crucial! It tells Docker to look for the Dockerfile in the current directory. This path is also known as the "build context" – any files in this directory can be copied into your image during the build process.
When you run this command, Docker will execute each instruction in your Dockerfile, creating a new layer for each step. You'll see output in your terminal as it progresses.

Once the build is complete, you can verify that your new image exists by running:

docker images

You should see my-custom-nginx listed there!

Does this explanation help you understand how to build your own Docker image? This is a more advanced topic than just pulling images, but it's where the real power of Docker customization comes in!