Docker is an open-source platform for containerization. If you haven't heard what a container is, docker.com defined it as follows:
A container image is a lightweight, stand-alone, executable package of a piece of software that includes everything needed to run it: code, runtime, system tools, system libraries, settings.
It's basically another way of virtualization on operating system level. It's more lightweight than a virtual machine because it basically uses the same kernel as the host OS. It creates an isolation for the virtual environment just by using namespaces and control groups.
We will take a look at how to create a container from a docker configuration file, which is called a Dockerfile. Dockerfile simply includes the set of commands that are needed to run to prepare your custom container. These configurations are then run to generate a docker image, and those images can be run to create containers.
Many organizations have already official images for their products. They are available on Docker Store and can easily be inherited by your custom configurations.
In my example, we will create an Ubuntu server container for Python development purposes. In order to follow the next steps, you must install Docker on your computer by simply following the official instructions.
You can clone my Dockerfiles from GitHub. You just need to create a folder, create a text file under it and call it "Dockerfile". This is the default name docker uses to build an image when docker build command is executed within a folder. Now let's fill your Dockerfile. First, we will use the official Ubuntu base image. To inherit other images, you use the statement FROM.
FROM ubuntu:latest
:latest is called a "tag" and it is used to check for a specific version defined in the image name, and you can also use other custom versions such as :17.10. These are image specific so each image may have different tags defined.
RUN apt-get update && apt-get install -y \
apt-utils \
git \
emacs-nox \
vim \
curl \
wget \
psmisc \
tmux \
software-properties-common
Another statement you will use a lot is RUN. This is for basically what you execute in a shell, also will be run when your image is getting built. There's a convention for adding each command or package as a new line so that it's easier to read. As you can see I have custom packages that I prefer such as text editors, or terminal management tool like tmux. You can modify those however you find it fit for your needs. Now let's install Python 3 and its dependencies.
RUN apt-get update && apt-get install -y \
python3 \
python3-pip \
python3-dev
I use pipenv for Python package dependency management, so I will install it using pip.
RUN pip3 install --upgrade pip && \
pip3 install pipenv
ENV SHELL /bin/bash
Pipenv requires SHELL environment variable so as you can guess, we define environment variables by ENV for our container. Now we will also set default locale for our container and our Dockerfile will be ready to be built.
RUN apt-get update && apt-get install -y locales && rm -rf /var/lib/apt/lists/* \
&& localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
ENV LANG en_US.utf8
This part specifically was taken from official Postgres image configuration, which is also recommended by docker.com itself.
Now we are ready to build our image. Simply go to the parent folder of Dockerfile in terminal and run the following command.
docker build -t myimage:v0.0.1 .
Beware of the . in the end, which is the path for your Dockerfile. This command will download inherited images from Docker Store if not available locally and execute the commands to get your image ready. -t flag is used to tag your image in name:tag format. You can use any custom format of your choice for tagging your images. Once complete, you can see your local docker images by the following command.
docker image ls
Now you are ready to run your first container.
docker run -it --name mycontainer myimage:v0.0.1
This is the very basic command to start a container with a defined image. It will instantly create a container from your image and attach you to it's shell. You will see something on your terminal like this:
$ docker run -it --name mycontainer myimage:v0.0.1
root@9ec80324ebeb:/#
This shows that you are logged in the container with ID 9ec80324ebeb as root user. To detach from a container, you can use keyboard shortcut Ctrl-p Ctrl-q. Be careful, if you type exit or use Ctrl-d it does not just detach from the container but also stops it. To list running containers simply run the following command:
docker ps
To include exited containers, you can run docker ps -a. If you accidentally exit a container, you can start it by:
docker start mycontainer
If you want to attach to a running container, you can do it with:
docker attach mycontainer
And to stop a container when you are not attached to it:
docker stop mycontainer
This is the very basic of how to start a container using a Dockerfile. If you have any questions, I am more than happy to help. Thanks for reading!