Editing Docker (section)

==Concepts==

* Docker runs on a Host Operating System.
* A '''Docker Container''' is a lightweight, standalone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries and settings, but in difference to VMs it does not include the (guest) OS layer (difference between Docker and VMs see [https://www.docker.com/resources/what-container here]). Containers isolate software from its environment and ensure that it works uniformly despite differences for instance between development and staging. Docker container have their own (temporary) file system. Docker Container are created when you run an image.
* A Docker Container is running on the '''Docker Host'''. Multiple container can run on the same host. The '''Docker Daemon''' as part of the host manages the containers
* A Docker Container can have the '''states''' 'created', 'running', 'restarting', 'exited', 'paused', 'dead'. There could be several reasons why a running container would exit. Let's look into a few of them:
** The process inside the container was completed, and so it exited.
** The process inside the container encountered an exception while running.
** A container is intentionally stopped using the docker stop command.
** No interactive terminal was set to a container running bash.
* The '''Docker Client''' steers the daemon and is accessed via terminal (Powershell, Linux shell, …).
* A '''Docker Image''' is a unmutable template with instructions for creating Docker containers and is build using a file called Docker file. The Docker image is stored in the Docker Hub or in a registry.
* Docker image ---> commands (pull from repository, …) ---> Docker container
* Images are stored in a registry, the public registry is '''Docker Hub'''.
* '''Docker Volumes''' are used to store data and to share data across containers.

===Creating Images===
# create a file called 'Dockerfile'. Dockerfiles define images.
# run the following command to build an image
 docker build Dockerfile -t <IMAGE_NAME>
 // docker push <IMAGE_NAME>


=====Dockerfile=====
Dockerfiles contain instructions. Comment lines starts with # (hash).

Docker files consists of <INSTRUCTION> <ARGUMENTS>. The first instruction is always
 FROM <BASE_IMAGE> // which defines the parent or base image e.g. FROM Ubuntu, by default Docker Hub is used as registry
Then the RUN is executed to install the application and packages which are required
 RUN apt-get install python
Then copy source code e.g.
 COPY . /app // app is a directory of the container 
The work directory defines the directory for the commands e.g.
 WORKDIR /app
To run something the RUN command is used e.g.
 RUN pip install flask
Define default command
 CMD ... // e.g. 'CMD sleep 5' or 'CMD ["sleep","5"]
or 
 ENTRYPOINT["<COMMAND"] // e.g. 'ENTRYPOINT ["sleep"]' and call it with 'docker run <IMAGE_NAME> [<ENTRYPOINT_PARAMETER>]

=====Analyzing Images=====
* there is a tool called 'dive' to analyze images

===Volumes===
* Volumes are folder on the host which are mounted to a container.

===Networks===
The network in charge of the communication process between containers is by default the '''bridge''' network or the user-defined bridge networks.
* host name from container <code>host.docker.internal</code>
* default networks [[File:DockerNetwork1.PNG|400px]]
* the docker engine has a internal DNS server [[File:DockerNetwork2.PNG|400px]]