Docker Cheat Sheet

This is the upstream file for the Docker Cheat Sheet in techbeatly(Yet to publish).

• Essential Docker Registry Commands
  • Private Docker Registry and Access
• Docker Commands
  • Image Handling
  • docker build
  • Running Containers
  • Docker Utilities
  • Cleaning Docker Environment
• Dockerfile
• Advanced Dockerfile Instructions
  • The RUN Instruction
  • The LABEL Instruction
  • The WORKDIR Instruction
  • The ENV Instruction
  • The USER Instruction
  • The VOLUME Instruction
• Building Images with the ONBUILD Instruction
• Points to Remember

Essential Docker Registry Commands

docker login -u USER_NAME -p TOKEN REGISTRY_URL
                                # before we push images, we need to
                                  login to docker registry.

docker login -u developer -p ${TOKEN} \
  docker-registry-default.apps.lab.example.com
                                # TOKEN can be get as TOKEN=$(oc whoami)

docker images --no-trunc --format ' ' --filter "dangling=true" --filter "before=IMAGE_ID"
                                # list image with format and
                                # using multiple filters

Private Docker Registry and Access

kubectl create secret docker-registry private-docker-cred \
    --docker-server=myregistry
    --docker-username=registry-user
    --docker-password=registry-password
    [email protected]
                                # Create a secret for docker-registry

Then specify the image pull secret under the imagePullSecrets of pod/deployment definition (same level of container)

    imagePullSecrets:
    - name: private-docker-cred

Docker Commands

Image Handling

docker create [IMAGE]           # Create a new container from a particular image.
docker search [term]            # Search the Docker Hub repository for a particular term.
docker login                    # Log into the Docker Hub repository.
docker pull [IMAGE]             # Pull an image from the Docker Hub repository.
docker push [username/image]    # Push an image to the Docker Hub repository.
docker tag [source] [target]    # Create a target tag or alias that refers to a source image.

docker build

docker build [OPTIONS] PATH
docker build --help

  -t, --tag - set the name and tag of the image
  -f, --file - set the name of the Dockerfile
  --build-arg - set build-time variables

Running Containers

docker start [CONTAINER]        # Start a particular container.
docker stop [CONTAINER]         # Stop a particular container.
docker exec -ti [CONTAINER] [command]
                                # Run a shell command inside a particular container.
docker run -ti — image [IMAGE] [CONTAINER] [command]
                                # Create and start a container at the same time, and then run a command inside it.
docker run -ti — rm — image [IMAGE] [CONTAINER] [command]
                                # Create and start a container at the same time,
                                # run a command inside it, and then remove the container
                                # after executing the command.
docker pause [CONTAINER]        # Pause all processes running within a particular container.

Docker Utilities

docker history [IMAGE]          # Display the history of a particular image.
docker ps                       # List all of the containers that are currently running.
docker version                  # Display the version of Docker that is currently installed on the system.
docker images                   # List all of the images that are currently stored on the system.
docker inspect [object]         # Display low-level information about a particular Docker object.

Cleaning Docker Environment

docker kill [CONTAINER]         # Kill a particular container.
docker kill $(docker ps -q)     # Kill all containers that are currently running.
docker rm [CONTAINER]           # Delete a particular container that is not currently running.
docker rm $(docker ps -a -q)    # Delete all containers that are not currently running.
docker network ls               # list available networks

Dockerfile

FROM - to set the base image RUN - to execute a command COPY & ADD - to copy files from host to the container CMD - to set the default command to execute when the container starts EXPOSE - to expose an application port

Advanced Dockerfile Instructions

The RUN Instruction

• RUN - executes the commands in a new layer on top of current image and then commits the result. (using /bin/sh to execute command)

  RUN yum --disablerepo=* --enablerepo="rhel-7-server-rpms"
  RUN yum update
  RUN yum install -y httpd
  RUN yum clean all -y
  

  • each RUN will create and additional layer and effects image size. So the best practice is to combine multiple RUN commands together if possible (use &&)

    RUN yum --disablerepo=* --enablerepo="rhel-7-server-rpms" && \
    yum update && \
    yum install -y httpd && \
    yum clean all -y
    

The LABEL Instruction

• LABEL - defines image metadata as key-value pair. OpenShift supported labels
  • io.openshift.tags
  • io.k8s.description

  • io.openshift.expose-services

    LABEL version="2.0" \
      description="This is an example container image" \
      creationDate="01-09-2017"
    

The WORKDIR Instruction

• WORKDIR - set the working directory for any following RUN, CMD, ENTRYPOINT, COPY, ADD instructions. Recommended to use absolute path.

The ENV Instruction

• ENV - defines the environment variables inside the container.
  • use env command to list environment variables inside a container.

  ENV MYSQL_DATABASE_USER="my_database_user" \
    MYSQL_DATABASE="my_database"
  

The USER Instruction

• USER - run image as a specific user (recommended to run as non-root user). OpenShift will ignore the USER instruction and will use a random userid other than root (0).

The VOLUME Instruction

• VOLUME - crate a mount point inside the container and keep the data persistent.

Building Images with the ONBUILD Instruction

• use ONBUILD to declare instructions that are executed only when building a child image.

FROM registry.access.redhat.com/rhscl/nodejs-6-rhel7
EXPOSE 3000
# Mandate that all Node.js apps use /usr/src/app as the main folder (APP_ROOT).
RUN mkdir -p /opt/app-root/
WORKDIR /opt/app-root

# Copy the package.json to APP_ROOT
ONBUILD COPY package.json /opt/app-root

# Install the dependencies
ONBUILD RUN npm install

# Copy the app source code to APP_ROOT
ONBUILD COPY src /opt/app-root

# Start node server on port 3000
CMD [ "npm", "start" ]

Points to Remember

• Docker was started as a project by a company called dotCloud, made available as open source in March 2013.
• Kubernetes surfaced from work at Google in 2014, and became the standard way of managing containers.