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What Docker is doing behind the scenes
Docker uses a client-and-daemon model. The Docker client sends commands to the Docker daemon, which manages Docker objects such as images, containers, networks, and volumes. You can think of the client as the control panel and the daemon as the part that does the work.
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Docker’s documentation defines an image as “a read-only template with instructions for creating a Docker container.” An image is not itself a running application; a container is created from the image, with its own configuration and writable layer. Docker’s overview of images and containers explains this distinction.
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A Dockerfile is the build recipe
A Dockerfile contains instructions for assembling an image. It describes what the image should include and how it should be prepared. It is not the running app, nor is it a list of services that should all start together.
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An image is the reusable template
Building from a Dockerfile produces an image. The image provides the filesystem and instructions used to create a container. The same image can be used to create more than one container.
A container is the configured instance
A container is created from an image and has runtime configuration, such as networking and port settings. It also has a writable container layer for changes made while it runs. That writable layer is temporary: if the container is removed, data stored only there is removed with it.
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What Docker Compose adds
One container may be enough for a simple task. An application with a web process and a database or cache has multiple services that need configuration and a way to communicate. A Compose file describes those services and their settings together; it is an application-level description, not a replacement for the images each service uses.
Compose can create the network that lets services in the same application communicate. Services can address each other over that shared network. The Compose application model describes services, networks, and volumes; Docker’s guide to what Compose is explains its role in defining and running multi-container applications.
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The common lifecycle commands are:
docker compose upstarts the application described by the Compose file.docker compose downstops and removes the running services created for that Compose application. Volumes are retained by default; adding the volumes option removes them as well and can erase persistent application data.
Why localhost may not reach a container
Being able to reach a service from the host is different from one container reaching another. Containers on a shared Docker network can communicate, but to reach a container service from the host, its port must be published. Docker’s container-running guide covers port publishing and container networking.
A published port maps a host port to a port inside the container. The numbers do not have to match. In Docker’s Compose Quickstart, host port 8000 maps to container port 5000. The app listens on port 5000 inside its container, while a browser on the host uses the published host-side port. If a local URL fails, check whether the service is listening, whether the port is published, and which host port the mapping exposes.
Why data disappears—and how to keep it
The container’s writable layer is useful for runtime changes, but it is not a safe place for data that must outlive the container. When a container is removed, changes kept only in that layer disappear. Store important data in a volume or another appropriate persistent location instead.
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- Named volume: Docker manages the storage separately from the container lifecycle, making it suitable for persistent container data. Compose retains volumes when you run
docker compose downby default. Adding the volumes option removes them too, so use it only when you intend to delete that data. - Bind mount: Shares a path from the host with a container. It is often useful during development when you want changes to host files to be available inside the container.
Docker’s running-containers documentation covers storage and container behavior. The practical question is where the data lives: in the removable container layer, in persistent storage, or in a host directory shared through a bind mount.
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How the pieces work together in Docker’s Compose example
Docker’s official Compose Quickstart uses a Flask web service and Redis. The Compose file describes both services so they can start as one application. It maps host port 8000 to the Flask service’s container port 5000, illustrating why the port entered in a browser can differ from the port the app uses internally.
The tutorial also demonstrates two issues that are easy to confuse with Docker’s image/container distinction. First, starting two services together does not necessarily mean Redis is ready when the web app tries to use it; the guide addresses that startup race by making the web service wait for Redis to become healthy. Second, Redis data that must survive container removal needs persistent storage, so the example uses a named volume. The example is a useful illustration of Compose configuration, readiness, networking, port mapping, and storage—not a claim that every application needs precisely the same setup.
For a broader guided sequence, Docker’s Getting Started tutorial covers containers, builds, volumes, networking, Compose, caching, and multi-stage builds.
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