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Testing Docker setup

Let's ensure that our Docker setup works perfectly. For our purpose, Docker Community Edition should suffice (https://www.docker.com/community-edition). Once we have it installed, we will check if it works by running a few basic commands.

Let's start by checking what version we have installed:

$ docker --version
Docker version 17.12.0-ce, build c97c6d6

Let's try to dig deeper into details about our Docker installation:

$ docker info
Containers: 38
 Running: 0
 Paused: 0
 Stopped: 38
Images: 24
Server Version: 17.12.0-ce

On Linux, when you try to run docker commands, you might get Permission denied error. In order to interact with Docker, you can either prefix the command with sudo or you can create a "docker" user group and add your user to this group. See link for more details https://docs.docker.com/install/linux/linux-postinstall/.

Let's try to run a Docker image. If you remember the discussion regarding the Docker registry, you know that we do not need to build a Docker image using Dockerfile, to run a Docker container. We can directly pull it from Docker Hub (the default Docker registry) and run the image as a container:

$ docker run docker/whalesay cowsay Welcome to GopherLand!  

Unable to find image 'docker/whalesay:latest' locally
Trying to pull repository docker.io/docker/whalesay ...
sha256:178598e51a26abbc958b8a2e48825c90bc22e641de3d31e18aaf55f3258ba93b: Pulling from docker.io/docker/whalesay
e190868d63f8: Pull complete
909cd34c6fd7: Pull complete
0b9bfabab7c1: Pull complete
a3ed95caeb02: Pull complete
00bf65475aba: Pull complete
c57b6bcc83e3: Pull complete
8978f6879e2f: Pull complete
8eed3712d2cf: Pull complete
Digest: sha256:178598e51a26abbc958b8a2e48825c90bc22e641de3d31e18aaf55f3258ba93b
Status: Downloaded newer image for docker.io/docker/whalesay:latest
 ________________________
< Welcome to GopherLand! >
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    \    
     \     
      \    
                    ##        .
              ## ## ##       ==
           ## ## ## ##      ===
       /""""""""""""""""___/ ===
  ~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ /  ===- ~~~
       \______ o          __/
         \            __/
            \__________/

The preceding command could also have been executed, as shown here though, merely using docker run ..., which is more convenient:

$ docker pull  docker/whalesay & docker run docker/whalesay cowsay Welcome to GopherLand!

Once we have a long set of built images, we can list them all and similarly for Docker containers:

$ docker images
REPOSITORY                         TAG            IMAGE ID            CREATED             SIZE
docker.io/docker/whalesay   latest         6b362a9f73eb    2 years ago         247 MB
$ docker container ls --all 
CONTAINER ID        IMAGE                COMMAND                  CREATED             STATUS                     PORTS               NAMES                                   
a1b1efb42130        docker/whalesay      "cowsay Welcome to..."   5 minutes ago       Exited (0) 5 minutes ago                       frosty_varahamihira

Finally, it is important to note that as we keep using docker to build and run images and containers, we will start creating a backlog of "dangling" images, which we might not really use again. However, they will end up eating storage space. In order to get rid of such "dangling" images, we can use the following command:

$ docker rmi --force 'docker images -q -f dangling=true'
# list of hashes for all deleted images.