Docker Basics¶

Docker packages applications into containers - isolated, portable runtime environments built from images. This entry covers images, containers, volumes, Dockerfiles, and common operational patterns.

Core Concepts¶

Concept	Description
Image	Immutable template for containers. Stored in registries.
Container	Running instance of an image. Can be started/stopped/deleted.
Volume	Persistent storage outside container filesystem. Survives `docker rm`.
Registry	Image repository (Docker Hub, GHCR, private).

Image naming: [registry/][namespace/]name[:tag] (e.g., nginx:latest, ghcr.io/org/repo:v1.2)

Images¶

docker pull ubuntu:20.04              # pull specific version
docker images                         # list local images
docker rmi ubuntu:20.04               # remove image
docker image prune                    # remove dangling images
docker image prune -a                 # remove ALL unused images

Containers¶

Run¶

docker run ubuntu echo "hello"        # run command and exit
docker run -it ubuntu /bin/bash       # interactive terminal
docker run -d -p 8080:80 --name web nginx  # detached, port mapping
docker run --rm -it ubuntu bash       # auto-remove on exit
docker run -e KEY=VALUE myapp         # with env variable
docker run -v /host:/container image  # bind mount
docker run -v myvolume:/data image    # named volume

Key flags: -i (interactive), -t (TTY), -d (detach), --rm (auto-remove), --name (name), -p (port), -v (volume), -e (env var)

Lifecycle¶

docker ps                             # running containers
docker ps -a                          # all containers
docker stop container                 # graceful stop
docker kill container                 # force stop
docker start container                # start stopped
docker restart container
docker rm container                   # remove stopped
docker rm -f container                # force remove
docker container prune                # remove all stopped

Interact¶

docker exec -it container bash        # shell in running container
docker exec container ls /app         # run command
docker logs container                 # view stdout/stderr
docker logs -f container              # follow logs
docker logs --tail 100 container      # last 100 lines
docker inspect container              # detailed JSON info
docker stats container                # live resource usage
docker top container                  # processes inside
docker cp file.txt container:/path/   # copy to container
docker cp container:/path/file.txt ./ # copy from container

Volumes¶

Named Volumes¶

docker volume create myvol
docker volume ls
docker volume inspect myvol
docker volume rm myvol
docker volume prune                   # remove unused

Bind Mounts¶

docker run -v /host/path:/container/path image
docker run -v $(pwd):/app image           # mount current dir
docker run -v $(pwd):/app:ro image        # read-only

Dockerfile¶

FROM ubuntu:20.04
WORKDIR /app
COPY . .
COPY requirements.txt /app/
RUN apt-get update && apt-get install -y python3 pip
RUN pip install -r requirements.txt
ENV PORT=8080
EXPOSE 8080
CMD ["python3", "app.py"]

# ENTRYPOINT + CMD pattern
ENTRYPOINT ["python3"]
CMD ["app.py"]     # default arg, overridable

Build¶

docker build -t myapp:latest .
docker build -t myapp:1.0 -f Dockerfile.prod .
docker tag myapp:latest registry/myapp:1.0
docker push registry/myapp:1.0

Cleanup¶

docker container prune                # stopped containers
docker image prune -a                 # unused images
docker volume prune                   # unused volumes
docker network prune                  # unused networks
docker system prune -a --volumes      # EVERYTHING unused
docker system df                      # disk usage

Patterns¶

# Stop all running containers
docker stop $(docker ps -q)

# Remove all containers
docker rm $(docker ps -aq)

# Remove all images
docker rmi $(docker images -q)

# Enter running container by name filter
docker exec -it $(docker ps -q -f name=myapp) bash

Deletion Order¶

When removing an image with containers: 1. docker stop container - stop 2. docker rm container - remove container 3. docker rmi image - remove image

Gotchas¶

Container filesystem is ephemeral - data lost on docker rm unless volumes are used
EXPOSE in Dockerfile is documentation only - does NOT publish the port; use -p at run time
CMD is overridden by docker run arguments; ENTRYPOINT is not (use --entrypoint to override)
Anonymous volumes get random names and are hard to manage - prefer named volumes
docker system prune does NOT remove named volumes - add --volumes flag
Build context (.) includes all files in current dir - use .dockerignore to exclude large files