Docker Images and Containers

Images & Containers

A Docker Image is like a blueprint or a snapshot of your application, including all the necessary dependencies and configurations.

A Docker Container is a running instance of this image. It is a lightweight, isolated runtime environment that uses the image as its base.

Using & Running External Images

docker run node # this node docker will exit automatically
docker run -it node # in this case, we can interact with the node docker
 
docker run --name some-mysql -e MYSQL_ROOT_PASSWORD=mysql -p 3306:3306 -d mysql:9.3.0

Build Image with a Dockerfile

FROM node
 
# by default the WORKDIR is /root
WORKDIR /app
 
copy . /app
 
RUN npm install
 
# this is optional, this doesn't really expose the port
EXPOSE 80
 
CMD ["node", "server.js"]

👉 RUN vs CMD

• RUN: Executes commands during image build time, creating a new layer in the image.
  • Example: RUN apt-get install -y nodejs (Installs Node.js in the image).
• CMD: Sets the default command to run when the container starts.
  • Example: CMD ["node", "app.js"] (Starts the Node.js app when the container runs).

👉 The command to build an image from Dockerfile

docker build -t myapp:latest $path_to_Dockerfile

👉 Image Layers

• Docker Image Layers: Each Docker image is composed of multiple layers, which are created sequentially from each instruction in the Dockerfile (like FROM, RUN, COPY).
• How Layers Work:
  • Each layer is a read-only snapshot of the filesystem.
  • Changes in one layer do not affect other layers.
  • Layers are cached, making image builds faster.
• Example:

# Dockerfile Example
FROM ubuntu:20.04          # Base layer (first layer)
RUN apt-get update         # Second layer
RUN apt-get install -y curl # Third layer
COPY . /app                # Fourth layer

• Layer Optimization:
  • Layers are reused if the instructions haven't changed.
  • Minimize the number of layers for a smaller image size.

Manage Images & Containers

💡 use --help to see all options

# list all images
docker images
 
# list containers that are running
docker ps
 
# list all containers including the stopped ones
docker ps -a
 
docker image inspect image_id

Stopping & Restarting Containers

docker start $container_name_or_container_id # start an stopped container, by default in detach mode
docker restart $container_name_or_container_id # restart an existing container
docker stop $container_name_or_container_id # stop an existing container
docker run # create a new container based on the image、

👉 docker run vs docker start

• docker run:
  • Creates a new container from an image.
  • Runs the container immediately.
  • Can accept options (e.g., environment variables, port mappings).
  • Example: docker run -d --name my_container my_image.
• docker start:
  • Starts an existing, stopped container.
  • Does not create a new container.
  • Only works with containers that have been previously created.
  • Example: docker start my_container.

Attached & Detached Containers

The detached mode is the default for running with docker start

The attached mode is the default for unning with docker run

docker container attach $container_name_or_container_id
docker logs $container_name_or_container_id # show the console output of container
docker logs -f $container_name_or_container_id # follow

Interactive Mode

Deleting Images & Containers

# remove container
docker rm container_name
 
# remove image
# the corresponding containers need to be removed first
docker rmi image_id
 
# remove unused images
docker image prune

Removing Stopped Containers Automatically

# automatically remove the container and its associated anonymous vlumns when it exits
docker run --rm image_id

Copying Files Into & From A Container

docker cp $path_from $path_to

Naming & Tagging Containers and Images

docker build -t image_name:tag . 
docker run --name $container_name $image_id
docker tag $old_name:$old_tag $new_name:$new_tag #rename a image

Pushing Images to DockerHub

1. Create a repo on docker hub
2. Push the local image to docker hub