Docker

Intro

Docker was pretty intimidating to me at first. After upgrading to Windows 10, installing Docker Desktop, and playing around with some of the tutorials it has started to make a bit more sense. First, there is a great cheat sheet available here:

• Docker Cheat Sheet

This includes a useful description of the docker run command and its main parameters:

docker run
            --rm   remove container automatically after it exits
             -it   connect the container to terminal
      --name web   name the container
      -p 5000:80   expose port 5000 from the container as port 80 on the local machine
  -v ~/dev:/code   create a host mapped volume inside the container
     alpine:3.4    the image from which the container is instantiated
        /bin/sh    the command to run inside the container

I've also seen some tutorials use the "-w" (as in "-w /opt") command. This specifies the working directory.

Terminology

I keep confusing docker terminology (especially "container" versus "image" and "repository" versus "registry"). Here are some key terms to keep me straight:

• Container - A running instance of an image
• Image - The basis for a running container. An image is an "ordered collection of file system changes" and execution parameters for running the image in a container
• Registry - A Registry is a hosted service containing repositories of images which responds to the Registry API.
• Repository - A repository is a set of Docker images. A repository can be shared by pushing it to a registry server. The different images in the repository can be labeled using tags.

For a complete docker glossary, see: https://docs.docker.com/glossary/.

Namespaces

Large companies might organize the repositories inside their registry into namespaces. This is a logical grouping that just uses part of the repository path to group repositories. As an example, a team's repository paths might look like:

docker-prod.registry.mycompanycom/myteam/awesome-image:1.2

This breaks down into:

registry/namespace/repository:tag

Online Articles

Here are some online tutorials to help get started:

Useful Commands

Connect to a Running Container

docker container exec -it [container_name] /bin/bash

This command assumes that "/bin/bash" is installed on the container. If present, it will be invoked. "-it" connects interactively to the terminal. Otherwise, the command would just exit.

Using Docker behind a Proxy

If your Dockerfile build scripts need to access the internet, they will need to do so through the proxy. Usually, it is enough to set the http_proxy and https_proxy environmental variables while building the image. This can be done as shown:

http_proxy=http://user:password@proxy.domain.com:port/
https_proxy=$http_proxy
docker build \
  --build-arg http_proxy=$http_proxy \
  --build-arg https_proxy=$https_proxy \
  -t <tag> \
  .

This can be helpful when building docker images on your laptop using public repositories. However, this isn't really what you're supposed to do. Instead, most companies will setup internal registries and repositories (such as Artifactory) to serve dependencies in the internet in a safer manner. These can be setup to proxy public repositories and serve the same content, but in a way that allows the company to see what artifacts are being pulled in from the internet, scan them to make sure they are safe, and sleep a little more soundly at night.

Automated Builds

The GitLab build script below is one example of how you can automate the compilation and pushing of docker images to a repository:

APP_NAME="docker-test"
BUILD_NUMBER=$(git describe --tags)
NAMESPACE="my-team"
DOCKER_REGISTRY="docker-prod.registry.my-company.com"
DOCKER_NAME=$NAMESPACE/$APP_NAME
DOCKER_IMAGE=$DOCKER_REGISTRY/$DOCKER_NAME
docker logout ${DOCKER_REGISTRY}
docker build --build-arg version=${BUILD_NUMBER} --file Dockerfile --tag ${DOCKER_IMAGE} .
docker tag ${DOCKER_IMAGE} ${DOCKER_IMAGE}:latest
docker tag ${DOCKER_IMAGE} ${DOCKER_IMAGE}:${BUILD_NUMBER}
docker images --all
docker login -u ${DOCKER_USERNAME} -p ${DOCKER_PASSWORD} ${DOCKER_REGISTRY}
docker push ${DOCKER_IMAGE}:${BUILD_NUMBER}
docker push ${DOCKER_IMAGE}:latest