Docker Vocabulary: The Essential Terms Every Developer Should Know in English

A read-only template that contains your application code, runtime, libraries, and configuration. Images are built from a Dockerfile and stored in a registry.

“Pull the latest image before running: docker pull node:20-alpine.”

A live, running instance created from an image. Containers are isolated from each other and from the host, but they share the host OS kernel.

“The container crashed on startup. Check the logs with docker logs <container-id>.”

Each instruction in a Dockerfile creates a layer. Layers are cached, so unchanged layers are reused on the next build, making builds faster.

“Install dependencies before copying source code so the dependency layer is cached.”

A text file with instructions for building a Docker image. Each line is a command: FROM, RUN, COPY, CMD, and so on.

“The Dockerfile is missing a CMD instruction. The container will start but do nothing.”

The default command run when a container starts. It can be overridden from the command line. Only the last CMD in a Dockerfile takes effect.

“We set CMD ["node", "server.js"] so the app starts automatically.”

Defines the executable that always runs when the container starts. Unlike CMD, it is not easily overridden. Often combined with CMD for default arguments.

“Use ENTRYPOINT ["python"] and CMD ["app.py"] to allow overriding the script without changing the interpreter.”

Docker reuses layers from previous builds when the source has not changed. Ordering instructions from least to most likely to change maximises cache hits.

“Move the COPY . . step after npm install so the cache is not busted on every code change.”

The set of files Docker sends to the daemon when you run docker build. By default it is the current directory. Keep it small with a .dockerignore file.

“The build context is 2 GB because node_modules is not in .dockerignore.”

A Dockerfile technique that uses multiple FROM statements. Each stage can copy artifacts from the previous one, producing a smaller final image with no build tools.

“We added a multi-stage build so the production image does not include the Go compiler.”

The EXPOSE instruction documents which port the container listens on. It does not publish the port to the host; that requires the -p flag at runtime.

“Add EXPOSE 3000 to the Dockerfile so developers know which port to map.”

A Docker-managed storage location outside the container filesystem. Volumes persist when the container stops or is removed, and can be shared between containers.

“Mount a volume for the database so data survives container restarts.”

Maps a directory from the host machine into the container. Changes on the host are immediately visible inside the container and vice versa. Common in development.

“Use a bind mount in development so code changes reload without rebuilding the image.”

Forwarding a host port to a container port using the -p flag. Format: -p host-port:container-port.

“Run with -p 8080:3000 to access the app at localhost:8080.”

Docker networks let containers communicate with each other by name. The default bridge network isolates containers from the host network.

“Create a custom network so the API container can reach the database container by name.”

A key-value pair passed to a container at runtime with -e or in a .env file. Used for configuration like database URLs and API keys.

“Pass DATABASE_URL as an environment variable instead of hardcoding it in the image.”

A secure way to pass sensitive data (passwords, tokens) to containers. Docker secrets are stored encrypted and mounted as files, not environment variables.

“Store the API key as a Docker secret so it never appears in the container's environment variables.”

A command Docker runs inside the container at regular intervals to determine if the container is healthy. An unhealthy container can be restarted automatically.

“Add a health check that curls /healthz so the orchestrator knows when the service is ready.”

The mechanism Docker uses to collect and route container logs. Options include json-file (default), syslog, Fluentd, and others.

“Set the log driver to Fluentd to stream container logs to our central logging system.”

A server that stores and distributes Docker images. Docker Hub is the default public registry. Teams also run private registries for internal images.

“Push the image to the private registry before deploying to production.”

The official public registry for Docker images. You can pull official images for databases, runtimes, and tools from Docker Hub without authentication.

“We use the official postgres image from Docker Hub instead of building our own.”

A label attached to an image to identify a specific version. Format: image-name:tag. If no tag is given, Docker uses 'latest' by default.

“Always pin a specific tag like node:20.11.0 instead of node:latest in production Dockerfiles.”

Uploading a local image to a registry with docker push. The image must be tagged with the registry URL before pushing.

“The CI pipeline builds the image and pushes it to ECR on every merge to main.”

Downloading an image from a registry to the local machine with docker pull.

“Run docker pull to get the latest image before running integration tests.”

Docker Compose is a tool for defining and running multi-container applications with a single docker-compose.yml file.

“We use Docker Compose locally to run the API, the database, and the cache together.”

In Docker Compose (and Swarm), a service is one container definition in the configuration. Each service can scale to multiple replicas.

“The compose file defines three services: api, db, and redis.”

The background process (dockerd) that manages containers, images, networks, and volumes on the host. The Docker CLI communicates with the daemon over a socket.

“The build failed because the Docker daemon was not running. Start it with systemctl start docker.”

Docker's built-in container orchestration mode. Swarm lets you manage a cluster of Docker hosts and deploy services with replication and rolling updates.

“We use Swarm for our on-premise deployment, but the cloud team uses Kubernetes.”

Automated management of containerised services across multiple hosts: scheduling, scaling, networking, and health monitoring. Kubernetes and Swarm are orchestration tools.

“At this scale we need a proper orchestration layer, not just Docker Compose.”