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.