What is a Dockerfile?

A Dockerfile is a text file that contains instructions for building a Docker image. It defines the steps needed to create a Docker image, including specifying the base image to use, adding dependencies, copying files into the image, setting environment variables, and specifying commands to run when the container is started. Dockerfiles allow developers to automate the process of building Docker images in a reproducible and efficient manner.

Example of a Dockerfile:

# Use an existing docker image as a base
FROM alpine:latest

# Set the working directory inside the container
WORKDIR /app

# Copy files from the host filesystem to the container filesystem
COPY . .

# Install dependencies (in this case, just a hypothetical package)
RUN apk add --no-cache some-package

# Expose a port for communication
EXPOSE 8080

# Define the command to run the application when the container starts
CMD ["./app"]

Explanation:

• FROM: Specifies the base image to build upon. In this case, it's Alpine Linux.
• WORKDIR: Sets the working directory inside the container where subsequent commands will be executed.
• COPY: Copies files from the host filesystem to the container filesystem.
• RUN: Executes commands in the container during the build process. Here, it installs a hypothetical package using the Alpine package manager (apk).
• EXPOSE: Exposes a port on the container. It doesn't actually publish the port, but it documents that the container listens on the specified port at runtime.
• CMD: Specifies the default command to run when the container starts. In this example, it runs the app executable.

With this Dockerfile, you can use the docker build command to build an image:

docker build -t my-image .

This command tells Docker to build an image using the Dockerfile in the current directory (.) and tag it with the name my-image.