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=== Part 2: ===

What is a Dockerfile?

A Dockerfile is a script containing a set of instructions used by Docker to automate the process of building a new container image. It defines the environment inside the container, installs necessary software, sets up commands, and more.

==== Basic Structure of a Dockerfile ====

A Dockerfile consists of a series of instructions and arguments. Each instruction is an operation used to build the image, like installing a software package or copying files. The instruction is written in uppercase, followed by its arguments.

==== Key Dockerfile Instructions ====

    `FROM`: Specifies the base image to start from. It's usually an OS or another application.
        Example: `FROM ubuntu:20.04`

    `LABEL`: Adds metadata to the image, like maintainer information.
        Example: `LABEL maintainer="name@example.com"`

    `RUN`: Executes commands in a new layer on top of the current image and commits the result.
        Example: `RUN apt-get update && apt-get install -y nginx`

    `CMD`: Provides defaults for the executing container. There can only be one CMD instruction in a Dockerfile.
        Example:` CMD ["nginx", "-g", "daemon off;"]`

    `ENTRYPOINT`: Configures the container to run as an executable. It's often used in combination with CMD.
        Example: `ENTRYPOINT ["nginx"]`

    `COPY`: Copies files or directories from the host machine to the container.
        Example: `COPY ./webapp /var/www/webapp`

    `ADD`: Similar to COPY, but can also handle URLs and tarball extraction.
        Example: `ADD https://example.com/app.tar.gz /app/`

    `WORKDIR`: Sets the working directory for any subsequent `RUN`, `CMD`, `ENTRYPOINT`,` COPY`, and `ADD` instructions.
        Example: `WORKDIR /app`

    `EXPOSE`: Informs Docker that the container listens on the specified network port at runtime.
        Example: `EXPOSE 80`

    `ENV`: Sets environment variables.
        Example: `ENV MY_VARIABLE=value`

    `VOLUME`: Creates a mount point for external storage or other containers.
        Example: `VOLUME /data`
 
===== Let's create a Dockerfile for a basic web server using Nginx: =====

First, create a folder called `my-webserver` and go inside it `cd my-webserver`

Then create another folder inside that called '''website''' and a file called '''index.html''' within the folder '''website''' with any content of your choice.
 
Create a file named '''dockerfile''' with the following content within the '''my-webserver''' folder. (ie. my-webserver/website/index.html and my-webserver/dockerfile)

{{{
# Use the official Nginx image as a base
FROM nginx:latest

# Set the maintainer label
LABEL maintainer="name@example.com"

# Copy static website files to the Nginx web directory
COPY ./website /usr/share/nginx/html

# Expose port 80 for the web server
EXPOSE 80

# Default command to run Nginx in the foreground
CMD ["nginx", "-g", "daemon off;"]
}}}

==== Building an Image from a Dockerfile ====

To build a Docker image from your Dockerfile, navigate to the directory containing the Dockerfile and run:

`docker build -t my-webserver:latest .`

This command tells Docker to build an image using the Dockerfile in the current directory (.) and tag it as `my-webserver:latest`.

Best Practices

•	Minimize Layers: Try to reduce the number of layers in your image to make it lightweight. For instance, chain commands using && in a single RUN instruction.
•	Use .dockerignore: Just like .gitignore, you can use .dockerignore to exclude files that aren't needed in the container.
•	Avoid Installing Unnecessary Packages: Only install the packages that are necessary to run your application.
•	Clean Up: Remove temporary files and caches to reduce image size.