Core Concepts

Master four core objects and you have Docker's skeleton: image, container, volume, network.

Image

An image is a read-only template containing everything needed to run an app. It is stored in layers, and identical lower layers are shared across images to save space.

• Source: pull from a registry (docker pull) or build from a Dockerfile (docker build).
• Analogy: like an "installation disc" — immutable, endlessly copyable.

Container

A container is a running instance of an image. You start processes inside it; the container adds a thin writable layer on top of the image's read-only layers.

• Lifecycle: create → run → stop → remove.
• Analogy: like a "system installed from the disc" — the disc (image) is unchanged, the installed system (container) can be modified.

💡 Data written inside a container is lost when the container is removed by default. Use a volume for persistence.

Volume

A volume is Docker-managed persistent storage, independent of the container lifecycle. Even if the container is deleted, the volume's data remains.

• Unlike a bind mount (host directory), a volume is created and managed by Docker, making it more portable across platforms.
• Typical uses: database files, config files, ROS workspace data.

Network

Networks let containers talk to each other and to the host. Docker provides several drivers:

Driver	Use
bridge (default)	Container communication on a single host
host	Container shares the host network stack directly
none	No network
custom bridge	Project-scoped isolation with container-name DNS

tip

How the four relate

        Registry
            │ pull / push
            ▼
         Image ──run──▶ Container
                         │ mount
                         ▼
                       Volume (persistence)
                         │ attach
                         ▼
                       Network (connectivity)

Install from Install Docker; command usage is in Commands.