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Docker packages applications and their dependencies into images, then runs those images as isolated containers. An image is the reusable, read-only template; a container is a running instance of that template. That distinction makes the basic workflow easier to understand: pull or build an image, start a container, inspect it, and stop or remove it when you are done.
What is Docker?
Docker is a platform for developing, shipping, and running applications. Its tools help package an application and the files and configuration it needs, distribute that package, and run it in a container. Docker’s beginner documentation introduces the platform and its core workflow.
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Containerization is the practice of packaging an application and its dependencies so it can run in an isolated container environment. Containers are not full virtual machines: a container is a process running with isolation and configuration supplied by the container platform. Packaging can make deployment more consistent, but it does not guarantee identical behavior on every operating system or remove security risks.
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A container is a runnable instance of an image. It has an isolated process tree and filesystem, along with configured networking and storage. A container can be started, stopped, and removed independently of the image it came from.
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When a container runs, it has a writable layer for changes made during its lifetime. Removing the container also removes changes held only in that layer. Data that must outlast the container needs persistent storage, such as a volume, rather than relying on the disposable layer.
What is a Docker image?
A Docker image is a read-only package used as a template to create containers. It contains the files and configuration needed to run an application and is built from layers. One image can be used to create multiple containers; each container gets its own writable layer.
| Aspect | Image | Container |
|---|---|---|
| Purpose | Read-only template for running an application | Runnable instance created from an image |
| Changes | Built as image layers | Runtime changes go into a writable layer |
| Lifecycle | Can be used to create multiple containers | Removing it discards unpersisted changes |
A registry stores and distributes images. Docker Hub is a public registry and is the default image lookup location in Docker’s documented default setup.
How do I run a Docker container?
Docker’s docker run command creates and starts a container from an image. If the image is not already local, Docker may fetch it from the configured registry. In this example, an NGINX image runs in the background and its container port 80 is published to port 8080 on the host:
docker pull nginx
docker run -d --name web -p 127.0.0.1:8080:80 nginxVisit http://127.0.0.1:8080 on the same computer to reach the published service. The -p option maps a host port to a container port. Binding to 127.0.0.1 makes this example accessible through the host’s loopback interface; if you omit a host IP, current Docker documentation says a published port binds on all interfaces by default. See Docker’s references for running containers and port publishing.
Useful commands for inspecting and cleaning up this example are:
docker ps
docker ps -a
docker logs web
docker stop web
docker rm webdocker pslists running containers; add-ato include stopped ones.docker logs webdisplays output from the container namedweb.docker stop webstops that container;docker rm webremoves it after it has stopped.
How do I build a Docker image?
To build an image, write a Dockerfile: a text file containing instructions for the image builder. These common instructions cover the base image, working directory, files to copy, build-time commands, and default command:
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteFROMselects a starting image.WORKDIRsets the working directory for later instructions.COPYcopies files from the build context into the image.RUNexecutes a command while the image is being built.CMDspecifies the default command when a container starts.
For a small example, create a file named Dockerfile in an empty project directory:
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FROM nginx
COPY site/ /usr/share/nginx/html/Put the site files in a site subdirectory, then run these commands from the directory containing the Dockerfile:
docker build -t my-app:1.0 .
docker run --rm -d --name my-app -p 127.0.0.1:8080:80 my-app:1.0Here, docker build reads the Dockerfile and builds an image tagged my-app:1.0. The final . is the build context: the current directory and the files the builder can use. The second command starts a container from the image, maps the service to the host’s loopback address, and removes the container when it stops because of --rm. The site directory must be inside the build context for the COPY instruction to access it. Docker explains these basics in its guides to writing a Dockerfile and build context.
What does EXPOSE do, and why might a port not be reachable?
The Dockerfile instruction EXPOSE documents a port the application is intended to listen on inside the container. It does not publish that port to the host. To make a service reachable from the host, publish a mapping when starting the container, for example -p 127.0.0.1:8080:80 for host port 8080 to container port 80. If the service still cannot be reached, check that the container is running, that the application listens on the expected container port, and that the mapping uses the right ports.
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Install Docker using the official installation instructions for your operating system; setup requirements differ by platform. Then follow Docker’s beginner path or hands-on workshop to practice starting containers, writing a Dockerfile, and building an image.
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