Docker is one of the newest kids on the block. Awhile ago, I posted about how you can get started with Vagrant to create isolated development environments. We’re going to do the same with Docker.
What’s the difference? To sum it up, Vagrant manages virtual machines and Docker manages application environments. Check out this Stackoverflow discussion with the authors of both systems chiming in.
- docker-machine create (optional) to create host machine
- docker build to build docker image
- docker run to instantiate a container from an image
- docker exec -i … bash to get into container
- docker commit (optional) to convert container to image
- docker push to upload image to Docker Hub
- docker pull to download an image
- docker save/load to save a committed image to a tar file
- docker stop to stop a container
- docker start to start a container
Ok so if you’re convinced you need to run Docker or you just want to add Docker to your skillset, let’s get started…
First get Docker. Go to docker’s website and download it. (I can’t link you directly to the download since I don’t know if you’re running Mac, Windows or Linux.)
It’s worth noting I have Mac OS X 10.10 and I ended up installing Docker 1.9.1 at the time of this writing. Your experience may be different but hopefully not by much.
Quick architecture lesson. Docker works off LinuX Containers (LXC). Mac and Windows are obviously not Linux so they work slightly differently. On my Mac, Docker requires a virtual machine, specifically VirtualBox to run a linux OS so Docker can work.
Here’s my understanding of the Docker architecture in my own image
At the foundation is the machine, or more precisely a Linux machine with a kernel that supports LXC’s. Docker lets you “build” an Image which contains all the layers of changes you made. A Container is an instance of an image. It’s the thing that’s runnable. A good description at Stackoverflow.
You get two tools after the installation
- Docker Quickstart Terminal (docker’s command-line or CLI): gives you a terminal initialized with the “default” host virtual machine settings. (I’ll comment more about this below.)
- Kitematic: a GUI to manage the running containers
Let’s talk about the Machine
Besides the two tools, you also get a Virtualbox (if you’re on a Mac or Windows). This is your docker-machine. You can see all the docker machines in your system by running this command in the Docker Quickstart Terminal CLI
> docker-machine ls
Your may get a listing that looks similar to this
NAME ACTIVE DRIVER STATE URL SWARM ERRORS
default * virtualbox Running tcp://192.168.99.100:2376
You likely have one machine called “default”. It’s the active one. I believe what makes it active depends on the environment variables in your terminal. To see what environment variables are set for the “default” machine type
> docker-machine env default
To switch to an active machine, you simply set the environment variables for that machine. You can use the linux command eval to run the output of the last command like so
> eval "$(docker-machine env default)"
You can remove and create machines using the docker-machine command. See all available commands by typing
> docker-machine help
Why would you need a different machine or new machine? I’m not really sure. But I had to remove my “default” one and recreate it again to add some dns settings like so
> docker-machine create -d virtualbox --engine-opt dns=126.96.36.199 default
For some reason, my docker containers didn’t have access to the internet due to bad DNS settings and this has to be set at the machine level.
There may be a less destructive way of altering the DNS according to this thread.
Now that you’re familiar with the machines, let’s move on.
Open the Docker Quickstart Terminal CLI and run
> docker images
You probably don’t have any images yet. Let’s get an image from Docker Hub.
> docker pull linode/lamp
This command fetches a docker image called linode/lamp which contains a LAMP stack.
If you run “docker images” again, you should now see the fetched image.
At this time, you should get acquainted with Docker Hub and create an account on it (remember your username). Docker Hub is a place where you and others can push docker images to share with the rest of the world. This includes pre-built LAMP stack images or it could just be a simple database image. As a user of docker, you may be able to just get away with using these pre-built images…. but we’re going to get our hands dirty and create our own docker image.
Create an empty directory, and in it, create a file called Dockerfile. Here’s a sample
# inherit from the baseimage ‘linode/lamp’
# Just a note about who wrote this image
# Since it’s a LAMP image, this exposes the the apache2 and mysql ports
EXPOSE 80 3306
Now in the same directory where the Dockerfile is, run
> docker build -t myname/myimage .
- docker build: command to build an image
- -t: flag to assign a repo and tag to the image
- myname: should be the account name you just created on Docker Hub
- myimage: is a name you want to assign to this image
- ‘.‘: to indicate the image should be built from the current directory where the Dockerfile is located
Check your list of images again with the “docker images” command and you should see your newly built image.
Now let’s talk about containers
Remember that a container is an instance of an image. So now that we’ve got an image, we can instantiate a container with the “docker run” command like so
> docker run -d -P myname/myimage
- docker run: command to create a container from an image
- -d runs the container in the background
- -P publishes all exposed ports (ie port 80 and 3306) to arbitrary ports available on the host
- myname/myimage is the image name that we’re instantiating
You should be able to see your running container by typing
> docker ps
You can also see your containers if you open Kitematic.
Inside Kitematic, look under Settings followed by Port.
There you’ll see the port mappings of the two exposed ports.
You might ask, how do I get into the container to view the directory, edit the config files or execute scripts?
From Kitematic, you can enter the container via the Exec button
This will open a shell script into the container. This shell lacks tab-completion, history and other basic shell niceties, but it gets you inside.
There’s another way in though. Open Docker CLI and type
> docker exec -it container_name bash
- docker exec executes a command inside the container
- -it runs the command in interactive mode with text console
- container_name is arbitrary name given to your container when you created it
- bash is the command you’re running which in this case is the bash shell
In summary, this (exec)utes an (-i)nteractive (bash) shell into your (container_name) container
This approach is superior since you get history and tab-complete in a familiar interactive bash shell.
Inside the container, go ahead and create a file or edit the filesystem in some way. Now you can take my word for it or try it yourself, but the data inside a container is essentially considered ephemeral. If you were to change the name of the container or some other minor change, everything would be wiped out. More importantly, you couldn’t transfer this container and its contents to anyone else unless you performed one additional step.
Commit your Docker container as an image
We’re going to convert our container into an image which can be persisted and transferred.
First we need the container ID (not the name but the nonsensical alphanumeric ID) from this command
> docker ps
Next commit your container
> docker commit CONTAINER_ID myname/myimage:version
- docker commit converts the container to an image
- CONTAINER_ID id of the container you’re converting
- myname name that should match your Docker Hub account name
- myimage name of image
- version version of image (e.g. 1.0 or “latest”)
Check your set of images again with “docker images”. You should now have an image that encapsulates the container and any changes to it.
Now you’re ready to share this image.
Push your docker image to Docker Hub
> docker push myname/myimage:version
- docker push uploads the image to Docker Hub
- myname/myimage:version is the image you’re pushing
Check your Docker Hub account and you should see your image. Now anyone can pull your image like we pulled linode/lamp earlier.
Save your docker image to a tar file (and Load it back)
Alternatively, you can save your image to a tar file instead of pushing it to Docker Hub. If you have private, proprietary data, you may not want it in the public Hub, or you may just want to transfer this image internally or to a client
> docker save myname/myimage:version > myimage.tar
To reload it
> docker load < myimage.tar
You should be able to see your loaded image with this command
> docker images
Start and Stop a container
You can list the available containers with this command
> docker ps -a
To start a container
> docker start [CONTAINER_ID]
To stop a container
> docker stop [CONTAINER_ID]