Docker Super Quickstart

20 Nov 2018:

A short introduction about the most important docker commands/functionality which might be useful for others.

Installing Docker

To install the community edition of docker on Ubuntu:

sudo apt-get remove docker docker-engine docker.io
sudo apt-get update
sudo apt-get install \
    apt-transport-https \
    ca-certificates \
    curl \
    software-properties-common
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"
sudo apt-get update
sudo apt-get install docker-ce

Running Docker containers

Here is a tutorial article about Docker with Ubuntu which is very helpful to understand the basics of docker.

Some commands:

  • docker ps - List all running Docker containers
  • docker container ls --all - List all containers that have been created locally
  • docker rm <name> - Use the name (from the command above) to delete a container.
  • docker stop <name> - As the command says
  • docker run -p 4000:80 <image> - Forward port 4000 on the host to port 80 on the guest

Building your own

To start your own Docker image you need to create a Dockerfile. This contains all steps needed. E.g. (taken from the official Docker documentation):

# Use an official Python runtime as a parent image
FROM python:2.7-slim

# Set the working directory to /app
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Define environment variable
ENV NAME World

# Run app.py when the container launches
CMD ["python", "app.py"]

If you, like me, like to put the Docker files all in their own subdirectory this command can be used to build the image:

docker build -f docker/Dockerfile -t <some-name-for-dockerimage> .

Investigate whats wrong at a certain build step. You can inspect the last one by using the last succeeded layers id

docker run --rm -it 829d658fa3b8 bash --login

829d658fa3b8 is the id shown in the last successful build step.

Running commands in a specific container

When the container is still running you can run commands in it by using

docker exec <container-name> <command>

If the last process exits from a container, the container will be stopped and enter the stopped state. Then it is not possible anymore to run commands in it. To rerun a container you need to make its state fixed and known to docker. You can do this with the commit subcommand:

docker commit <container-id>

You can find the container id with docker container ls --all

It will then create a new container state and output its layer id. You can use the same run command as above when creating and inspecting the build of the Dockerfile:

docker run -it 829d658fa3b8 bash --login

Substitute 829d658fa3b8 with the output ID from the commit command.

This time we left the --rm away, so the container is not deleted on exit and can be used to create a new layer with commit.

Mounting folders as Volumes from the Host

When running a container you can bind a folder from the host computer to a folder in the container with -v. The format is as follows

-v <localfolder>:<guestfolder>

So e.g. with

-v "$PWD":/build

you will mount the current directory from the host to /build in the container. This Option can be passed with the run subcommand.

Saving and loading for deployment

Once you are satisfied with a container you developed on your machine, you can save it to a file for deployment on some infrastructure.

docker save IMAGE_NAME | gzip > IMAGE_NAME.tgz

Loading from a file goes analoguous in reverse with load:

docker load < IMAGE_NAME.tgz

decompression is done and detected automatically.

Cleanup

Here some helpful commands for cleaning up after playing around:

  • docker stop $(docker ps -a -q) - Stop all docker containers
  • docker rm $(docker ps -a -q) - Remove all docker containers