Building Containerized Applications with DockerFile: Syntax, Examples and Use-Cases
Understanding Dockerfile and its Significance
Docker is a containerization platform that allows the development, testing, and deployment of applications within containers. Docker containers are lightweight and can be run anywhere, making them perfect for creating scalable, distributed applications. Docker uses a set of standard instructions written in a file known as Dockerfile, which can be used to automate the creation of Docker images.
A Dockerfile is a script containing a set of instructions that are used to build Docker images. The Dockerfile contains all the information required to create a complete and self-contained Docker image, from which containers can be launched. These instructions include the base image to be used, the commands to be executed, and the files to be copied into the image.
Syntax of Dockerfile
Dockerfile consists of a series of commands that are used to build a Docker image. Each command performs a specific function required for the construction of the image. Below is the basic syntax of Dockerfile:
# Comment
INSTRUCTION arguments
Each instruction and its arguments form a layer in the container, allowing changes to be done at each layer.
Dockerfile Examples
Example 1
The following example creates a Dockerfile that installs Python 3.7 and Flask:
# Use an official Python runtime as a parent image
FROM python:3.7-slim-buster
# Set the working directory to /app
WORKDIR /app
# Copy the current directory contents into the container at /app
COPY . /app
# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt
# Make port 80 available to the world outside this container
EXPOSE 80
# Define environment variable
ENV NAME World
# Run app.py when the container launches
CMD ["python", "app.py"]
In this example, the Dockerfile starts with a parent image that is official Python 3.7 runtime. In the second line, the WORKDIR command is used to set the ‘/app’ directory as the working directory.
In the third line, the COPY command is executed to copy the contents of the directory into the container.
The fourth line installs the required packages using pip.
The fifth line exposes the container's port 80 to the outside world.
The sixth line sets the environment variable 'NAME' as 'World'.
Finally, the seventh line is used to specify the command that should be executed when the container starts.
Example 2
The following example creates a Dockerfile that runs a simple Node.js application:
# Use an official Node.js runtime as a parent image
FROM node:carbon
# Set the working directory to /app
WORKDIR /app
# Copy package.json and package-lock.json
COPY package*.json ./
# Install dependencies
RUN npm install
# Copy project files
COPY . .
# Expose port 3000
EXPOSE 3000
# Run the app
CMD ["npm", "start"]
In this example, we start with the official Node.js runtime image. In the second line, the WORKDIR command is used to set the '/app' directory as the working directory.
The third line is used to copy the package.json and package-lock.json files into the container. This is done separately from the rest of the project files to make use of Docker's caching mechanism.
The fourth line installs the dependencies using npm. This layer is also cached, so if there are no changes to the package.json or package-lock.json files, the cache is used, and the layer is not rebuilt.
The fifth line copies the project files into the container.
In the sixth line, port 3000 is exposed.
Finally, in the seventh line, the command to be executed when the container starts is defined.
Dockerfile Use-Case Scenarios
Dockerfile is used to create customized Docker images to run applications. Some of the use-case scenarios are:
Customized Development Environment
Dockerfiles can be used to create customized development environments for various programming languages. It helps developers build images that have all the necessary dependencies pre-installed.
CI/CD
Dockerfiles can be used to create images that can be deployed to Kubernetes clusters, which is useful in continuous integration and continuous delivery pipelines.
Microservices
Dockerfiles can be used to create Docker images of microservices that can be deployed in a distributed architecture.
Conclusion
Dockerfile is an essential tool for creating Docker images. It contains a set of instructions that define how the Docker image should be built. The Dockerfile is used to automate the Docker image creation process, making it repeatable and consistent every time. The examples above illustrate the different elements of a Dockerfile and how it can be used to create custom Docker images for various scenarios.