Containerization isolates different environments, allowing engineers to focus on development without spending excessive effort on environment setup.
Docker is currently a mature and popular containerization technology.

Exploring Docker Commands

View Container Information

docker ps List all running containers
docker ps -a List all existing containers

View Logs

docker logs [CONTAINER] Print logs
docker logs -f [CONTAINER] Continuously print the latest logs

Run Containers

Create a container from a local environment image. If it doesn’t exist, access the repository.
A “login” action is required for private repos. The image format can be [IMAGE NAME]:[TAG]

docker run

Examples:

# Run nginx
docker run -idt nginx

# Run nginx and expose port 8080
docker run -idt -p 8080:80 nginx

# Run nginx on port 8080, automatically restart on failure
docker run -idt -p 8080:80 --restart on-failure nginx

# Run nginx and remove after termination
docker run --rm -idt nginx

Commonly Used Flags

• -d detach: Run the container in the background
• -i interactive: Keep STDIN open, allowing interaction
• -t allocate a pseudo-TTY: Allow the container to use a TTY
• -p expose socket: Host Port:Docker Port (e.g., -p 8080:80)
• --restart: Restart policy (always, none, on-failure)
• --rm: Automatically remove the container after closing; otherwise, it may accumulate even if it’s dead

Common Commands

Enter a running container and open a terminal command interface

docker exec -ti [CONTAINER] bash

Get metadata of a container or image

docker inspect [CONTAINER]

Delete Docker container

docker rm [CONTAINER]

Delete Docker image

docker rmi [IMAGE]

View running processes in a container

docker top [CONTAINER]

Other Commands

Build Docker image using Dockerfile -f: Manually specify the Dockerfile name

# Build image in the current directory using Dockerfile
docker build . -t image-name

# Build image in the current directory using Dockerfile-alt
docker build . -t image-name -f Dockerfile-alt

Pull/Push docker image from/to repository Login to Docker Hub first

docker push / pull

Login to Docker repository

docker login

How to Write Your Dockerfile

• Register on Docker Hub first
• Alpine Linux
  • Very small size (5MB)
  • Complete Linux architecture
  • Rich component support

Common Dockerfile Commands

Basic

• FROM: Specify the base image; defaults to latest if no version is specified
• RUN:
  • Commands executed during docker build to install dependencies.
  • Each RUN is a separate event.
• ARG:
  • Build parameters during container docker build.
  • Cannot be used in containers.
• ENV:
  • Environment variables used inside the container.
  • Format: ENV <key> <value> or ENV <key1>=<value1> <key2>=<value2>

File Copy

• COPY:
  • Copy “source file/directory” to “file/directory” in the container
  • COPY [--chown=<user>:<group>] <source path>... <dist path>
  • Dist path can be either an absolute path inside the Container or relative path to the WORKDIR
  • Preserves various attributes of the source data
• ADD:
  • Enhanced version of COPY
  • ADD [--chown=<user>:<group>] <source path>... <dist path>
  • Allows source path to be a URL
  • File permissions are automatically set to 600

Execution

• ENTRYPOINT:
  • Default command executed when the container starts
  • The last command, should keep the container running in the foreground
• CMD:
  • Default command (or parameters) executed when the container starts
  • If both ENTRYPOINT and CMD exist, CMD is used as a parameter

FROM ubuntu

ENTRYPOINY ["ping"]
CMD ["localhost"]

WORKDIR(cd + mkdir):

• Initial working directory inside the container

Command Differences

ARG vs ENV

ARG is passed during image build

docker build . --build-arg FOO=bar

ENV is used as environment variables inside the Docker container

CMD vs ENTRYPOINT

ENTRYPOINT is executed by default when the container starts CMD is used as parameters if ENTRYPOINT exists

FROM ubuntu

ENTRYPOINY ["ping"]
CMD ["localhost"]

COPY vs ADD

COPY is used to copy files from the local machine to the container ADD is used to copy files from remote locations to the container

Debugging

docker: Got permission denied

This error occurs because the current user lacks permissions. You can elevate the user to root to ensure permissions.

However, a safer and correct approach is to:

1. Create a docker execution group
2. Add the user to the docker group

Commands below, need to logout and login again to take effect!

sudo groupadd docker
sudo usermod -aG docker user_name