Learn Docker with Hack

Overview

If you are interested in getting a basic understanding of what Docker is, then you are in the right place. Below you will find summaries and relevant commands for the major principles of Docker.

In addition, a detailed tutorial will allow you to put some of this material into practice. The tutorial walks through dockerizing the flask application in this repo. It will cover creating a Dockerfile, building an image, volumes, and networking.

Virtual Workshop

Youtube | Slides

Table of Contents

• What is Docker?
• Prerequisites and Setup
• Docker Principles and Commands
• Tutorial
• Additional Material

What is Docker?

Docker is an open platform for developing, shipping, and running applications. Docker enables you to separate your applications from your infrastructure so you can deliver software quickly. With Docker, you can manage your infrastructure in the same ways you manage your applications.

Prerequisites and Setup

Mac or Windows Install Docker Desktop

Docker Desktop is an easy-to-install application for your Mac or Windows environment that enables you to build and share containerized applications and microservices. Docker Desktop includes Docker Engine, Docker CLI client, Docker Compose, Notary, Kubernetes, and Credential Helper.

Linux Install Docker Engine

Docker Engine is an open source containerization technology for building and containerizing your applications. Docker Engine acts as a client-server application with:

• A server with a long-running daemon process dockerd.
• APIs which specify interfaces that programs can use to talk to and instruct the Docker daemon.
• A command line interface (CLI) client docker.

Docker Engine can be installed with a simple shell script as well. Run the following command in terminal.

curl -sSL https://get.docker.com/ | sh

Docker Principles and Commands

• Images
• Dockerfile
• Docker Registry
• Containers
• Volumes
• Networking

Images

An image is a read-only template with instructions for creating a Docker container. Often, an image is based on another image, with some additional customization. For example, you may build an image which is based on the ubuntu image, but installs the Apache web server and your application, as well as the configuration details needed to make your application run.

Image Commands

• docker build or docker image build Creates image from Dockerfile.
• docker image ls Shows all images.
• docker image import Creates an image from a tarball.
• docker image commit Creates image from a container, pausing it temporarily if it is running.
• docker image rm Removes an image.
• docker image load Loads an image from a tar archive as STDIN, including images and tags.
• docker image save Saves an image to a tar archive stream to STDOUT with all parent layers, tags & versions.

Dockerfile

To build your own image, you create a Dockerfile with a simple syntax for defining the steps needed to create the image and run it. Each instruction in a Dockerfile creates a layer in the image. When you change the Dockerfile and rebuild the image, only those layers which have changed are rebuilt. This is part of what makes images so lightweight, small, and fast, when compared to other virtualization technologies.

Possible Dockerfile Instructions

• FROM Sets the Base Image for subsequent instructions.
• RUN Execute any commands in a new layer on top of the current image and commit the results.
• CMD Provide defaults for an executing container.
• EXPOSE Informs Docker that the container listens on the specified network ports at runtime. NOTE: does not actually publish ports to host.
• ENV Sets environment variable.
• ADD Copies new files, directories or remote file to image. Invalidates caches. Avoid ADD and use COPY instead.
• COPY Copies new files or directories to image. By default this copies as root regardless of the USER/WORKDIR settings. Use --chown=<user>:<group> to give ownership to another user/group. (Same for ADD.)
• ENTRYPOINT Configures a container that will run as an executable.
• VOLUME Creates a mount point for externally mounted volumes or other containers.
• USER Sets the user name for following RUN / CMD / ENTRYPOINT commands.
• WORKDIR Sets the working directory.
• ARG Defines a build-time variable.
• ONBUILD Adds a trigger instruction when the image is used as the base for another build.
• STOPSIGNAL Sets the system call signal that will be sent to the container to exit.
• LABEL Apply key/value metadata to your images, containers, or daemons.
• SHELL Override default shell is used by docker to run commands.
• HEALTHCHECK Tells docker how to test a container to check that it is still working.

Docker Registry

A Docker registry stores Docker images. Docker Hub is a public registry that anyone can use, and Docker is configured to look for images on Docker Hub by default. You can even run your own private registry.

When you use the docker pull or docker run commands, the required images are pulled from your configured registry. When you use the docker push command, your image is pushed to your configured registry.

Registry Commands

• docker login To login to a registry.
• docker logout To logout from a registry.
• docker search Searches registry for image.
• docker pull Pulls an image from registry to local machine.
• docker push Pushes an image to the registry from local machine.

Containers

A container is a runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI. You can connect a container to one or more networks, attach storage to it, or even create a new image based on its current state.

By default, a container is relatively well isolated from other containers and its host machine. You can control how isolated a container’s network, storage, or other underlying subsystems are from other containers or from the host machine.

A container is defined by its image as well as any configuration options you provide to it when you create or start it. When a container is removed, any changes to its state that are not stored in persistent storage disappear.

Container Commands

Lifecycle

• docker run Creates and starts a container in one operation. (docker run --rm will remove the container after it stops)
• docker container create Creates a container but does not start it.
• docker container rename Allows the container to be renamed.
• docker container rm Deletes a container.
• docker container update Updates a container's resource limits.
• docker container start Starts a container so it is running.
• docker container stop Stops a running container.
• docker container restart Stops and starts a container.
• docker container pause Pauses a running container, "freezing" it in place.
• docker container unpause Will unpause a running container.
• docker container wait Blocks until running container stops.
• docker container kill Sends a SIGKILL to a running container.
• docker container attach Will connect to a running container.

Info

• docker ps Shows running containers.
• docker logs Gets logs from container.
• docker inspect Looks at all the info on a container (including IP address).
• docker events Gets events from container.
• docker port Shows public facing port of container.
• docker top Shows running processes in container.
• docker stats Shows containers' resource usage statistics.
• docker diff Shows changed files in the container's FS.

Import / Export

• docker container cp Copies files or folders between a container and the local filesystem.
• docker container export Turns container filesystem into tarball archive stream to STDOUT.

Executing Commands

• docker exec Execute a command in container. (To enter a running container, attach a new shell process to a running container, use: docker exec -it <container_id> /bin/bash)

Volumes

Volumes are the preferred mechanism for persisting data generated by and used by Docker containers. While bind mounts are dependent on the directory structure and OS of the host machine, volumes are completely managed by Docker.

Volume Commands

• docker volume create Create a new volume.
• docker volume rm Delete a volume.
• docker volume ls List volumes.
• docker volume inspect Display detailed information on one or more volumes.

Networking

Docker’s networking subsystem is pluggable, using drivers. Several drivers exist by default, and provide core networking functionality:

• bridge: The default network driver. If you don’t specify a driver, this is the type of network you are creating. Bridge networks are usually used when your applications run in standalone containers that need to communicate. See bridge networks.

• host: For standalone containers, remove network isolation between the container and the Docker host, and use the host’s networking directly. See use the host network.

• overlay: Overlay networks connect multiple Docker daemons together and enable swarm services to communicate with each other. You can also use overlay networks to facilitate communication between a swarm service and a standalone container, or between two standalone containers on different Docker daemons. This strategy removes the need to do OS-level routing between these containers. See overlay networks.

Network Commands

• docker network create Create a new network (default type: bridge).
• docker network rm Remove one or more networks by name or identifier. No containers can be connected to the network when deleting it.
• docker network ls List networks
• docker network inspect Display detailed information on one or more networks.
• docker network connect Connect a container to a network
• docker network disconnect Disconnect a container from a network

Tutorial

Overview

In this tutorial, we are going to deploy a simple flask backend using docker. We will need two containers, one for the database and one for the flask app. We will also need to make use of volumes, so our database data can persist, and networking, so that our containers can talk to each other.

Make sure that you have all necessary software installed before proceeding. Instructions can be found here

Steps

• Clone Repo
• Create a Dockerfile
• Build an Image
• Create a Network
• Create a Volume
• Spin Up Containers
• Test Deployment

Step 1: Clone Repo

• Clone this repository from GitHub in order to copy all the source code to your local machine.
• Open the repository in an editor. Get ready to add to the file named Dockerfile.

git clone https://github.com/zpinto/learn-docker-with-hack.git
code learn-docker-with-hack # if you use VSCode

Step 2: Create a Dockerfile

• Now that you have your Dockerfile opened up, it is time to write the instructions that will build your image.

• See if you can do it yourself using the information provided on Dockerfile instructions here. Once you have given it a try, verify your file with the snippet in the dropdown.

• What the Dockerfile should do:

  • Start with base image python:3.9
  • Expose PORT 80 so your flask app can listen on it
  • Copy only the requirements.txt into the container at /app/requirements.txt
  • Change your working directory to /app
  • Run pip install requirements.txt in order to install the app's dependencies
  • Copy the flask app into the container at /app
  • Set the entrypoint to run the command gunicorn --bind 0.0.0.0:80 wsgi:app

Correct Dockerfile Code

  FROM python:3.9

  EXPOSE 80

  # We copy just the requirements.txt first to leverage Docker cache
  COPY ./requirements.txt /app/requirements.txt

  WORKDIR /app

  RUN pip install -r requirements.txt

  # Assuming dependencies do not change, everything above will not have to be rerun when rebuilding the image
  COPY . /app

  ENTRYPOINT [ "gunicorn", "--bind", "0.0.0.0:80", "wsgi:app" ]

Step 3: Build an Image

• Now that our Dockerfile is ready to go, we can build our image.
• Our image needs a name and a tag.
• Run the build command from within the same directory as the Dockerfile to create an image called learn-docker-backend.

docker build -t learn-docker-backend:latest .

Step 4: Create a Network

• Now we need to create a network for our containers to talk to each other on.
• Our container containing the flask app will need to talk to the container running mongoDB
• Run the following command to create a bridge network called hack-net

docker network create hack-net

Step 5: Create a Volume

• In order to make sure that our database data persists passed the life of our mongoDB container, we need to create a volume that is managed by docker.
• We will later mount this to our container.
• Run the following command to create a volume named learn-docker-data

docker volume create learn-docker-data

Step 6: Spin up Containers

• Now it is time to start up our containers.
• Since our flask app depends on mongoDB, we need to create our mongoDB container first.
• Open a terminal window and run the following command to spin up a mongoDB container on the network we created and mounted to our volume we created.

docker container run --rm --name hack-mongo --network hack-net --volume learn-docker-data:/data/db mongo:latest

• Open a new terminal window in order to spin up our flask app container using the image we previously build from our Dockerfile.
• In addition to adding this container to the hack-net network, we will pass an environment variable into this container so that it knows the URI for the mongoDB instance. We will also publish port 80 of our container to port 5000 of our local machine so we can talk to the flask app from outside of the bridge network we created.
• Run the following command to create a container from the image learn-docker-backend.

docker container run --rm --network hack-net -e MONGO_URI_MASTER="mongodb://hack-mongo:27017/zothacks2020"  -p 5000:80 learn-docker-backend:latest

• The following should give guidance as to what all args and flags are.

docker container run --rm --name <container-name> --network <connected-networks> -e <environment-variable>=<value>  -p <publish-port-localhost>:<publish-port-container> --volume <volume-name>:<container-mount-point> <image-name>:<version-number>

Step 7: Test Deployment

• We can now test out our flask app by using postman to make some requests.
• Follow the GIFs below.
• Import the postman collection from the repo.

• Create an environment in postman and add a variable called url. Set it to localhost:5000

• Make some requests to the flask app. If the requests work, you have successfully deployed the app.

Step 8: Clean Up

• In order to clean up and stop the containers, we will first find their IDs by running the following command.

docker container ls

• Now we will stop these containers by running the following command. (containers will also be removed since they were started with --rm option)

docker container stop <container_id>

• Remove the docker network and volume by running the following two command.

docker network rm hack-net
docker volume rm learn-docker-data

Additional Material

• Docker Docs
• Docker Class I Took