Step 2 of 4: Docker Images

Goal

In this step, you will search for, pull and import a few publicly available Docker images. You will also experiment with running a few simple commands on these images.

Pull an image

One of the features of Docker is that a lot of people have created Docker images for a variety of purposes. Many of these have been uploaded to Docker Hub, which is a publicly available registry where docker users can push their images and pull down the images others have made. We will pull down the Busybox image, which is a small linux distribution. You can download it using the docker pull command.

xnat@xnat-31:~$ docker pull busybox

This will find and download the busybox image from Docker Hub. You can now use this image by running a container.

xnat@xnat-31:~$ docker run -t busybox ls /bin
[                  freeramdisk        mkfifo             sha512sum
[[                 fsck               mkfs.ext2          showkey
acpid              fsck.minix         mkfs.minix         shuf
add-shell          fstrim             mkfs.vfat          slattach
addgroup           fsync              mknod              sleep
<output snipped>
find               mesg               setuidgid          yes
findfs             microcom           sh                 zcat
flock              mkdir              sha1sum            zcip
fold               mkdosfs            sha256sum
free               mke2fs             sha3sum

In this example:

• docker run runs a container.
• busybox is the image you would like to run.
• -t flag assigns a pseudo-tty or terminal inside the new container.
• The command /ls bin lists the contents of the /bin directory inside our container. 

We can see that the ls /bin command we just ran executed inside a container. That means that, essentially, a lightweight virtual machine based on the busybox image was started up just for our command to run, then shut down again when the command finished. This all happened in a fraction of a second, which is orders of magnitude faster than typical VMs. We can verify that the contents of the /bin directory on our XNAT VM are different from the contents of the container we just ran.

xnat@xnat-31:~$ ls /bin
bash			      	dir	     		loadkeys	       	ntfsfix				ss
bunzip2		      		dmesg	     	login	       		ntfsinfo			static-sh
busybox		      		dnsdomainname  	loginctl	       	ntfsls				stty
bzcat		      		domainname     	lowntfs-3g        	ntfsmftalloc		su
bzcmp		      		dumpkeys	    ls		       		ntfsmove			sync
bzdiff		      		echo	     	lsblk	       		ntfstruncate		tailf
<output snipped>
dbus-cleanup-sockets  	lessecho	    ntfscat	       		setfont				zgrep
dbus-daemon	      		lessfile	    ntfsck	       		setupcon			zless
dbus-uuidgen	      	lesskey	     	ntfscluster       	sh					zmore
dd		      			lesspipe	    ntfscmp	       		sh.distrib			znew
df		      			ln	     		ntfsdump_logfile  	sleep

The contents of /bin are completely different from what we saw inside the busybox container. 

The important concepts to take away from this are:

• Commands we run "inside a container" can be run identically from any machine. They will all see absolutely nothing about the machine running docker; the commands only see the container, which is based on a docker image, which is the same everywhere.
• Containers can be created from images almost instantaneously. This is different from virtual machines, which take a long time to copy and run.

Import Local Image Tar Archive

Although it is convenient to store and pull your images to a Docker repository like Docker Hub, in some instances you may wish save images to (and load images from) a local file.  We will use the docker load command to stream data from a prepackaged tar file located on your Vagrant VM.

xnat@xnat-31:~$ docker load -i /resources/docker/docker_ubuntu_latest.tar
9436069b92a3: Loading layer 127.6 MB/127.6 MB
19429b698a22: Loading layer 14.85 kB/14.85 kB
82b57dbc5385: Loading layer 11.78 kB/11.78 kB
737f40e80b7f: Loading layer 4.608 kB/4.608 kB
5f70bf18a086: Loading layer 1.024 kB/1.024 kB

In this example:

• docker load loads an image from a tar file
• -i is used to specify an input file, instead of STDIN
• ubuntu.tar is a (provided) file that we generated for you using docker save

View Installed Images

We now have several images installed on our Docker server.  View a list of these images.

xnat@xnat-31:~$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ubuntu              latest              2fa927b5cdd3        9 days ago          122 MB
busybox             latest              47bcc53f74dc        11 weeks ago        1.113 MB

Next we will see some more things you can do to run and manage docker containers.

Step 3: Docker containers