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Docker Objects

Updated Mar 11, 2022 ·

Overview​

Docker objects are the essential components that make up a Docker environment, facilitating application deployment and management.

Dockerfile​

A Dockerfile is a series of instructions for creating a Docker image:

  • Packages code and dependencies.
  • Each command creates a layer that is cached.
  • Only modified layers are rebuilt when changes occur.

For a complete list of instructions, refer to the official Docker documentation. Common instructions include:

  • FROM - Specifies the base image.
  • RUN - Executes a command during image build.
  • COPY and ADD - Transfer files from the host to the container.
  • CMD - Default command to run when the container starts.
  • EXPOSE - Opens a port for communication.

Here’s an example Dockerfile for a Python hello-world application:

# Set the base image
FROM python:3.8

# Set the maintainer label
LABEL maintainer="Eden Jose"

# Copy files to the container
COPY . /app

# Set the working directory
WORKDIR /app

# Install dependencies
RUN pip install -r requirements.txt

# Command to run on container start
CMD [ "python", "app.py" ]

Creating the Dockerfile​

Before creating the image, outline the steps to deploy the application. For example, deploying a web application might involve:

  1. Start with OS - CentOS
  2. Update repo
  3. Install dependencies
  4. Install Python packages
  5. Copy source code to /opt
  6. Run the web server

With these steps, you can begin containerizing your application:

  1. Create a Dockerfile.
  2. Build your image using the Dockerfile.
  3. Push it to a Docker registry.

Here's a basic flow for creating a Dockerfile:

Docker Image​

After creating the Dockerfile, you can build a Docker image, which is:

  • A read-only template.
  • Composed of an overlay filesystem.
  • Used to run container instances.

Overlay Filesystem​

A Docker image consists of multiple layers, starting with a base image and adding installed packages on top. The last layer is writable; data written here will be lost once the container stops unless specific storage options are set.

Building the Docker Image​

To build a Docker image from a Dockerfile, use the docker build command:

docker build [OPTIONS] PATH

To see all valid options for the build command, use:

docker build --help

For example, to build a Python "Hello-world" application from the current directory:

docker build -t python-helloworld .

To build from a different directory, use:

docker build -t python-helloworld /another/directory/python-app

To list all available images:

docker images