Docker Basics and Dockerfiles

Why Docker in a blockchain courseSearch on YouTubeBookmarkSaved

The syllabus lists Docker under "Base technologies" in the Introduction unit. Blockchain nodes, wallets and dev tools are usually shipped as containers. The lab uses Docker to run tools without installing them on the host.

Docker packages and runs an application in a loosely isolated environment called a container. The isolation lets you run many containers at once on one host. A container holds everything the app needs, so it does not depend on what the host has installed. Everyone who runs the same container gets the same behaviour.

Installing DockerSearch on YouTubeBookmarkSaved

Install Docker Desktop from the Docker docs ("Install Docker Desktop on Mac" or "on Windows"). On Windows it runs on WSL 2. Check both after install:

docker --version     # e.g. Docker version 29.6.2, build dfc4efb
wsl --version        # Windows only: WSL version, kernel version, Windows version

Image commandsSearch on YouTubeBookmarkSaved

docker pullSearch on YouTubeBookmarkSaved

Syntax: docker pull [OPTIONS] NAME[:TAG|@DIGEST]. It downloads an image from a registry (usually Docker Hub) to the local machine.

docker pull alpine
docker pull ubuntu:24.04
docker pull nginx:latest

• It downloads an image, not a container.
• If the image already exists locally, only missing or updated layers are downloaded.
• The default tag is latest if no tag is given (output: "Using default tag: latest").
• Images are stored in Docker's local image cache.
• The output shows the layer IDs, a Digest: sha256:... and the full name docker.io/library/alpine:latest.

docker imagesSearch on YouTubeBookmarkSaved

Syntax: docker images [OPTIONS] [REPOSITORY[:TAG]]. Both forms list local images:

docker images
docker image ls

Columns: IMAGE, ID, DISK USAGE, CONTENT SIZE, EXTRA. For example alpine:latest is about 8.42 MB.

Container lifecycleSearch on YouTubeBookmarkSaved

Listing containersSearch on YouTubeBookmarkSaved

docker ps        # running containers only
docker ps -a     # all containers (running + stopped); same as --all
docker ps -q     # container IDs only
docker ps -n 5   # last 5 created containers

Columns: CONTAINER ID, IMAGE, COMMAND, CREATED, STATUS, PORTS, NAMES.

Creating a containerSearch on YouTubeBookmarkSaved

docker create makes a container from an image but does not start it. Its STATUS is Created.

docker create --name cont1 alpine
docker create --name myalpine alpine
docker create --name web nginx
docker create --name test ubuntu sleep infinity

Starting a containerSearch on YouTubeBookmarkSaved

docker start cont1
docker ps -a     # STATUS: Exited (0) 10 seconds ago

cont1 starts and exits at once. Its default command is /bin/sh, which has no terminal attached, so it ends with exit code 0. A later docker exec cont1 pwd fails with "container ... is not running".

Keeping a container aliveSearch on YouTubeBookmarkSaved

Give it a main process that never ends:

docker run --name cont3 -d alpine sleep infinity
docker ps -a     # cont3: COMMAND "sleep infinity", STATUS Up 14 seconds

docker run = create + start in one step. -d runs it detached (in the background).

Running commands inside: docker execSearch on YouTubeBookmarkSaved

docker exec cont3 pwd          # prints /
docker exec cont3 ls           # bin dev etc home lib media mnt opt proc root ...
docker exec -t cont3 ls        # -t allocates a TTY, so output is in columns and coloured
docker exec -it cont3 /bin/sh  # interactive shell inside the container

Inside the Alpine shell, /bin holds BusyBox applets (ls, ping, sh, wget and so on).

Stopping and removingSearch on YouTubeBookmarkSaved

docker stop cont3      # graceful stop; status becomes Exited
docker rm cont3        # fails if running: "stop the container before removing or force remove"
docker rm -f cont3     # force remove a running container
docker rm e67c         # a unique prefix of the container ID works
docker rm cont1

Removing imagesSearch on YouTubeBookmarkSaved

docker rmi alpine ubuntu   # untags and deletes each layer (sha256 lines)
docker images              # list is now empty
docker image prune         # remove all unused images

Dockerfile basicsSearch on YouTubeBookmarkSaved

A Dockerfile lists the steps that build an image. Each instruction makes a layer.

FROM ubuntu
RUN apt-get update && apt-get install -y
CMD ["echo", "Welcome to Ubuntu Docker container!"]

docker build -t mycont1 .

-t tags (names) the image. The final . is the build context: the folder whose files are sent to the builder. By default Docker looks for a file named Dockerfile in that folder.

Custom Dockerfile namesSearch on YouTubeBookmarkSaved

Use -f to build from a file with another name, such as customalpine.txt:

FROM alpine
RUN apk update
RUN apk add --no-cache bash
CMD ["echo", "welcome to Custom Alpine"]

docker build -f customalpine.txt -t mycont2 .

apk is Alpine's package manager. --no-cache skips storing the package index, which keeps the image small.

docker historySearch on YouTubeBookmarkSaved

docker history <image> shows the layers of an image, newest first, with the instruction that created each one and its size.

docker history ubuntu
docker history mycont1
docker history alpine
docker history mycont2

For mycont2, the top three layers are the ones the custom file added (CMD 0 B, apk add 2.15 MB, apk update 3.05 MB). Below them sit the base Alpine layers (ADD alpine-minirootfs ... 8.42MB). Metadata instructions like CMD add 0 B layers.

Containerising a simple appSearch on YouTubeBookmarkSaved

The lab builds a single-page HTML site served by nginx.

# Dockerfile2
FROM nginx:alpine

WORKDIR /usr/share/nginx/html

COPY index.html .

EXPOSE 80

CMD ["nginx", "-g", "daemon off;"]

docker build -f Dockerfile2 -t mysite .
docker run --name mysite mysite

daemon off; keeps nginx in the foreground, so nginx stays PID 1 and the container keeps running.

Port mappingSearch on YouTubeBookmarkSaved

The container runs and docker ps shows PORTS 80/tcp. The host browser still shows "localhost refused to connect".

docker run -d -p 8080:80 --name mysite1 mysite

-p <host_port>:<container_port>. Now docker ps shows 0.0.0.0:8080->80/tcp, [::]:8080->80/tcp, and http://localhost:8080 loads the site.

More Dockerfile instructionsSearch on YouTubeBookmarkSaved

WORKDIRSearch on YouTubeBookmarkSaved

Sets the working directory inside the image for the following instructions. It creates the folder if missing.

FROM ubuntu:24.04
WORKDIR /app
COPY . .
RUN ls

COPY . . copies the build context into /app.

ENVSearch on YouTubeBookmarkSaved

Sets environment variables inside the image and container. Syntax ENV <KEY>=<VALUE>.

FROM ubuntu:24.04
ENV APP_ENV=production
ENV PORT=8080
WORKDIR /app

LABELSearch on YouTubeBookmarkSaved

Adds metadata to an image to identify, organise and manage it. Syntax LABEL key="value".

LABEL app="payment-service" \
      version="2.1.0" \
      team="backend" \
      environment="production"

ENTRYPOINTSearch on YouTubeBookmarkSaved

Defines the main executable the container is designed to run. Syntax ENTRYPOINT ["executable", "argument"].

FROM ubuntu:24.04
RUN apt-get update && apt-get install -y nmap
ENTRYPOINT ["nmap"]

Arguments given to docker run are appended to the entrypoint:

ENTRYPOINT fixes the executable. The user supplies the arguments at run time.

ENTRYPOINT vs CMDSearch on YouTubeBookmarkSaved

Use them together: ENTRYPOINT is the fixed program, CMD gives default arguments that the user can override.

FROM python:3.12
WORKDIR /app
COPY . .
ENTRYPOINT ["python"]
CMD ["app.py"]

Exec form vs shell formSearch on YouTubeBookmarkSaved

ENTRYPOINT python app.py         # shell form
CMD python app.py                # shell form
ENTRYPOINT ["python", "app.py"]  # exec form (JSON array)

Shell form makes Docker run /bin/sh -c "python app.py". The process tree is:

PID 1
└── /bin/sh -c python app.py
    └── python app.py

Your Python app is not PID 1. The shell is. Signals such as SIGTERM from docker stop go to the shell, not to your app. Exec form runs the app directly as PID 1:

PID 1
└── python app.py

Docker CLI cheat sheetSearch on YouTubeBookmarkSaved

Useful links from the sheet: docs.docker.com/desktop (install), github.com/docker/awesome-compose (example projects), docs.docker.com (docs).

Quick self-testSearch on YouTubeBookmarkSaved