Programming
A searchable guide to Docker images, containers, Dockerfile instructions, Docker Compose, ports, volumes, logs, and common troubleshooting steps.
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Core concepts
Docker
docker --versionA tool for packaging and running applications in isolated containers.
Use it when you want the app to run with the same OS packages, runtime, and dependencies across machines.
docker --versionExplain whether this project can be run with Docker.
Core concepts
Docker daemon
Cannot connect to the Docker daemonThe background service that builds images and runs containers.
If Docker commands cannot connect, Docker Desktop or the Docker service may not be running.
Cannot connect to the Docker daemonCheck why Docker commands cannot connect to the daemon.
Core concepts
Image
node:20-alpineA reusable template used to create containers.
Images define what OS, runtime, libraries, and app files a container starts with.
node:20-alpineTell me which base image this Dockerfile uses.
Core concepts
Container
docker psA running or stopped instance created from an image.
Use containers to run the app, database, cache, or worker in isolated environments.
docker psList running containers and explain what each one does.
Core concepts
Registry
docker pull postgres:16A server where Docker images are stored and downloaded from.
Docker Hub and private registries are used to share base images or deployable app images.
docker pull postgres:16Explain where this image will be downloaded from.
Dockerfile
Dockerfile
DockerfileA file that defines how to build a Docker image.
Use it to install dependencies, copy app files, and define the command that starts the app.
DockerfileRead this Dockerfile and summarize the build steps.
Dockerfile
FROM
FROM node:20-alpineThe Dockerfile instruction that chooses the base image.
It decides the starting OS layer and often the runtime version.
Changing the base image can affect available commands, security updates, and image size.
FROM node:20-alpineExplain what this FROM image includes.
Dockerfile
WORKDIR
WORKDIR /appSets the folder inside the image where following commands run.
Use it so COPY, RUN, and CMD commands operate from a predictable app folder.
WORKDIR /appExplain which folder this Dockerfile uses as the app directory.
Dockerfile
COPY
COPY package.json package-lock.json ./Copies files from the build context into the image.
Use it to add dependency files, source code, and static assets to the image.
COPY package.json package-lock.json ./Tell me which files this COPY step adds to the image.
Dockerfile
RUN
RUN npm ciRuns a command while building the image.
Use it to install OS packages, install dependencies, or build app files.
RUN happens at image build time, not when the container starts.
RUN npm ciSeparate Dockerfile build-time commands from container startup commands.
Dockerfile
CMD
CMD ["npm", "run", "start"]Defines the default command that runs when the container starts.
Use it to start the web server, API server, worker, or script.
CMD ["npm", "run", "start"]Tell me what command this container starts with.
Dockerfile
EXPOSE
EXPOSE 3000Documents which port the containerized app is expected to listen on.
Use it as a hint, then publish the port with docker run -p or compose ports.
EXPOSE alone does not make the app reachable from your browser.
EXPOSE 3000Explain whether this exposed port is actually published.
Dockerfile
.dockerignore
node_modulesA file that excludes paths from the Docker build context.
Use it to keep local dependencies, build output, secrets, and temporary files out of the image build.
node_modulesCheck whether this .dockerignore excludes unsafe or unnecessary files.
Dockerfile
Build context
docker build -t my-app .The folder sent to Docker during image build.
The final dot in docker build usually means the current folder is the build context.
A large context slows builds and can accidentally include private files.
docker build -t my-app .Explain what files are available to this Docker build.
Docker Compose
Docker Compose
docker compose upA tool for running multiple Docker services from one YAML file.
Use it for app plus database, cache, worker, or local development stacks.
docker compose upExplain what services this compose file starts.
Docker Compose
services
services: web: db:Named app parts defined in a compose file.
A web service might run the app while a db service runs PostgreSQL.
services: web: db:List the services in this compose file and what each does.
Docker Compose
docker compose up
docker compose up -dStarts the services defined in a compose file.
Use -d when you want services to run in the background.
docker compose up -dStart the compose services and show which ones are running.
Docker Compose
docker compose down
docker compose downStops and removes containers created by compose.
Use it when you are done with a local development stack.
Named volumes usually remain unless you explicitly remove volumes.
docker compose downStop this compose stack without deleting persistent data.
Commands
docker build
docker build -t my-app .Builds a Docker image from a Dockerfile and context.
Use it after writing a Dockerfile or changing app dependencies.
docker build -t my-app .Build this Docker image and summarize any build errors.
Commands
docker run
docker run --name my-app -p 3000:3000 my-appCreates and starts a container from an image.
Use it when you want to start one container directly without compose.
docker run --name my-app -p 3000:3000 my-appRun this image and explain the browser URL.
Commands
docker ps
docker psLists running containers.
Use docker ps -a when you also want stopped containers.
docker psShow running containers and their published ports.
Commands
docker stop
docker stop my-appStops a running container.
Use it before removing a container or freeing a port.
docker stop my-appStop the container using port 3000.
Commands
docker logs
docker logs my-appPrints logs from a container.
Use it when a container exits, crashes, or the app does not respond.
docker logs my-appRead this container's logs and find the first error.
Commands
docker exec
docker exec -it my-app shRuns a command inside an already running container.
Use it to inspect files, environment variables, or installed tools inside a container.
Changes made manually inside a container can disappear when the container is recreated.
docker exec -it my-app shOpen a shell in this container and check its environment variables.
Ports and network
ports
3000:3000Maps a port on your machine to a port inside the container.
Use it so a browser can reach a web app running inside a container.
The left side is the host port, and the right side is the container port.
3000:3000Explain which URL I should open for this ports setting.
Ports and network
localhost in Docker
host.docker.internallocalhost can mean the container itself, not your host machine, when code runs inside Docker.
Use service names in compose networks, or host.docker.internal when a container must reach the host machine.
host.docker.internalExplain why localhost does not reach the database from this container.
Files and data
Volume
db-data:/var/lib/postgresql/dataDocker-managed storage that can persist data outside a container's lifecycle.
Use volumes for database data or other files that should survive container recreation.
Removing volumes can delete persistent data.
db-data:/var/lib/postgresql/dataTell me which compose volumes store persistent data.
Files and data
Bind mount
./src:/app/srcConnects a host folder directly into a container path.
Use it during development so code edits on the host appear inside the container.
A bind mount can hide files that were copied into the image at the same path.
./src:/app/srcExplain whether this volume is a named volume or a bind mount.
Files and data
environment
DATABASE_URL=postgres://db:5432/appConfiguration values passed into a container.
Use environment variables for database URLs, ports, modes, feature flags, and runtime settings.
Do not bake secrets into public images or commit real secret values.
DATABASE_URL=postgres://db:5432/appCheck which environment variables this container needs.
Troubleshooting
port is already allocated
Bind for 0.0.0.0:3000 failed: port is already allocatedDocker cannot publish a host port because another process or container is already using it.
Find the process or container using the port, then stop it or choose a different host port.
Bind for 0.0.0.0:3000 failed: port is already allocatedFind what is using this Docker port and suggest a safe fix.
Troubleshooting
container name is already in use
Conflict. The container name "/my-app" is already in useA container with the same name already exists.
List existing containers, then reuse, remove, or rename the old container.
Conflict. The container name "/my-app" is already in useResolve this container name conflict without deleting data unexpectedly.
Troubleshooting
Build cache
docker build --no-cache -t my-app .Docker can reuse previous build layers to make builds faster.
If a change seems ignored, inspect the Dockerfile order or rebuild without cache as a diagnostic step.
--no-cache can make builds slower, so use it intentionally.
docker build --no-cache -t my-app .Explain whether Docker build cache could be hiding my change.
Safety
docker system prune
docker system pruneRemoves unused Docker data such as stopped containers, unused networks, and dangling images.
Use it when Docker uses too much disk space and you understand what will be removed.
With extra flags, prune can delete images or volumes you still care about.
docker system pruneBefore pruning Docker, show what would be removed and what data is safe.