Docker

Source: Dev.to

What is Docker

A Docker container is a running instance of a Docker image. It includes the application, libraries, and runtime, isolated from the host system.

Why use Docker

• Consistency – same environment on your laptop, CI, and production.
• Portability – images run anywhere the Docker engine runs.
• Efficiency – containers share the host kernel, start fast and use less memory than VMs.
• Simplifies CI/CD & microservices – build, ship, and scale small independent services easily.

Core concepts

• Image – immutable blueprint (read‑only).
• Container – running instance of an image (writable layer on top).
• Registry – storage for images (e.g., Docker Hub).
• Dockerfile – text file with build instructions for an image.
• Volume / Bind mount – ways to persist or share data outside a container.
• Network drivers – bridge (default), host, none, overlay (for multi‑host).
• Layer caching – build reuses unchanged layers to speed up builds.

Minimal Dockerfile — what each line means

FROM openjdk:17-jdk-slim      # base image with Java runtime
WORKDIR /app                  # set working directory inside the image
COPY . /app                   # copy files from host into the image
EXPOSE 8080                   # documents a port (does not publish it)
ENTRYPOINT ["java","-jar","app.jar"]  # executable used when container starts

• FROM – specifies the base image.
• WORKDIR – directory where subsequent commands run.
• COPY – copies files from the host into the image.
• EXPOSE – declares the port the container listens on (for documentation).
• ENTRYPOINT – defines the command that runs when the container starts.

Tip: add a .dockerignore to exclude build artifacts you don’t want in the image.

Essential commands (clean cheat‑sheet)

General info

docker --version            # show Docker client version
docker info                 # daemon summary (containers, images, storage driver)

Build images

docker build -t myapp:1.0 .                     # build image from Dockerfile in current dir
docker build --no-cache -t myapp:1.0 .         # rebuild without cache

Run & lifecycle (create, start, stop, remove)

docker run --name web -d -p 8080:80 nginx:latest
# -d (detached), -p hostPort:containerPort, --name containerName

docker run -it --rm ubuntu bash
# -it interactive, --rm remove on exit

docker ps                     # list running containers
docker ps -a                  # list all containers (including stopped)
docker stop       # graceful stop
docker start      # start stopped container
docker restart    # restart
docker rm          # remove container

Images

docker pull nginx:latest    # download image from registry

Inspect, exec & logs

docker logs -f           # follow logs
docker exec -it  bash    # open a shell inside a running container
docker inspect           # detailed low‑level information

Volumes & mounts (persist data)

docker volume create myvol
docker run -v myvol:/data ...                # named volume
docker run -v /host/path:/data ...           # bind mount
docker volume ls
docker volume rm myvol

• Named volumes (managed by Docker) are best for databases and portability.
• Bind mounts map a host folder directly into the container (useful for development).

Networking (quick)

docker network ls
docker network create mynet
docker run --network mynet ...
docker network inspect mynet

Cleanup (free disk space)

docker system df
docker image prune          # remove dangling images
docker container prune      # remove stopped containers
docker system prune -a      # remove unused images/containers/networks (use with care)

Compose (multi‑container apps)

docker compose up -d
docker compose down
docker compose ps
docker compose logs -f

Monitoring & debug

docker stats                   # live resource usage per container