Running a container from a Docker image

In the previous post I pulled three Docker images. 

If you are familiar with machine virtualization think of these as templates.  An image is used as the base of a container.

So, when a container is 'run' it is essentially a differencing file system that is linked to an image file system.  This allows something to happen within the container and for that to be written back to disk in a unique place.

This is really no different than the concept to using a single virtual disk and creating multiple virtual machines from it using differencing disks.  The differencing disk contains the unique character of each machine.  In this case the container contains any uniqueness.

Let me give a bit of background here.  If you were not aware, I am employed as a software tester.  I also do some development.  But, I like to keep my workstations clean.  And containers give me that in a tidy way.
Lately, I am working with a NodeJS application.  And I don't want to have to install Node and all of its dependencies on my workstation.  This is how I see many developers get themselves into the classic 'works on my machine' trap. 
At some point in time they installed some strange DLL or package ( or security settings or anything ) and then later took that as a dependency and never realized it.  Then they distribute the package, I put it on a different system, and wham, no workie.
So I am actually going to do a few tricks that containers easily enable.  And I will also use a real example that I am working with and you could as well.

Enough of that.  Lets take a moment to look at the syntax of the container run command
( the entire run reference is here:  http://docs.docker.com/reference/run/ )

I am going to begin with the Node image that I pulled down.  This image is based on Ubuntu and already has NodeJS and the NPM package manager installed.  This saves me a bit of time with installing Node, its dependent libraries

sudo docker run -i -t --name meshblu node:latest /bin/bash
Lets break this apart:

1. sudo - this is Ubuntu and Docker runs as root (or local system) so in order to interact with it, you must elevate yourself.  Windows admins; think RunAs.
2. docker - call the docker service then pass it a command
3. run - self explanatory, but I want to run a container
4. -i  - interactive.  This will put the container console into my current console.
5. -t  - virtual tty.  This gives you a tty console session for STDIN.  Important if you want to interact with the container in a console way.
6. --name  - this will be the name of the container, and simply helps you to keep them straight.  Without it a random (and silly) name is given.  And you will have to keep them straight.  The key here is that this is just a quick way to locate the container ID, which is the really important thing.
7. node:latest - this is the name of the image I pulled / want to use.  It will check if you have this local, if not it will look to the configured hub and try to find it and pull it.
8. /bin/bash - this is the command plus any arguments to run.  Everything after the image name will be executed within the container.  So you can have any command plus parameters at the end.  /bin/bash is simply a bash shell - pretty much were you are at any Linux console.

Go ahead, execute the command.
Notice that your prompt changed.  Because now your command window is connected to the container process and you are essentially "within" the container.  For me this is:  root@06b874873e86:/#
I am in the container as root, and the container ID just happens to be '06b874873e86'.

Now.  Exiting.
If you type 'exit' the container stops running and you drop back to your command prompt.
If you type 'ctrl+p' then 'ctrl+q' you drop back to your command prompt, but the container stays running.

To see any running containers type (at the Docker host):  sudo docker ps
To see all containers (running and stopped / paused): sudo docker ps -a
If you want back into a running container use:  sudo docker attach 06b87
(notice that I did not type the entire container id,  just enough to uniquely identify it.  A nifty usability feature)
Lastly, to start a container that has been stopped:  sudo docker start -i 06b87
(the -i connects you to it interactively)

Pulling Docker images

We have some Docker basics, and ran a bash command inside of a container.  And you possibly poked around a bit more.

After downloading that Ubuntu image on demand, you may have noticed that it looks like you have multiple images locally.  Where you really have the single image shown by the IMAGE ID but with multiple tags.

If you wonder where these images magically come from, it is a place called the Docker Hub ( hub.docker.com ).  Go to the hub and look around.  Notice that there are 'official' images and community images.  I, personally, stick with the official images as I know who is behind that image creation - Canonical is the source of the 'official' Ubuntu image.
Accountability, I like that.

Now I want a few images, I don't want to run them straight off, I want to download some official images and have them locally and then do some other things with them.  Also, this way I have them for offline use.

If you look at Ubuntu in the Docker Library ( https://registry.hub.docker.com/_/ubuntu/ ) you will notice the supported tags section.  In the previous post I referenced ubuntu:latest - looking at the tags you can see that this translates to trusty, and trusty ( I just happen to know ) is 14.04 LTS.

I could also pull Ubuntu 14.10 by defining ubuntu:utopic or get really experimental and use ubuntu:vivid

This is handy for many developers as they can define a version dependency, no different than a specific version of a DLL or a module.  Test can stabilize on a specific OS release, and so on.

So, lets pull the Mongo, Redis, and Node images.  Since I need a base MongoDB server, a Redis Server, and a place to run my NodeJS application.  This way I can work with these offline from Docker hub.

First node.  sudo docker pull node:latest
Notice that multiple images were downloaded.  At the time I wrote this there were 11.
All of these together will form the resulting image.  Kind of like using differencing disks and making changes and linking them together - one builds upon the previous.

After the download is complete, take a look  sudo docker images
And you see one image id.

If you want to know what is happening under the hood in Docker itself.  I found an excellent explanation to save me a bunch of typing: http://blog.thoward37.me/articles/where-are-docker-images-stored/
Now, the file locations are relative, but no longer exact due to updates to Docker.

But, as you can see from that post, this is Linux, so everything is simply right there on the file system of the Ubuntu Docker host.  Like a folder of files.  Not contained within some virtual disk ( which could be secured with permissions or BitLocker ). 
This is why we consider the host that runs Docker to be a physical security boundary and the running containers more of a process / network boundary.

Virtual Machines in themselves are considered physical security boundaries.  And the hypervisor system is designed to support and enforce that.

I will get deeper into that in a post or two just to show what you can do with this.  Basically, play a few virtualization tricks.

I had mentioned also pulling MongoDB and Redis; so lets go ahead and do that:
sudo docker pull redis:latest
sudo docker pull mongo:latest

At this point in time we should have pulled all of the images.  And next time we will do something more interesting.