Docker Images

• Docker images are required to create containers.
• From one Docker image, we can create multiple containers
• Images are build-time constructs and containers are run-time constructs.
• Let pull some images and observe the stuff as shown below
  
  
• Docker image is collection of Image Layers
  
• When we build the image we use some base image and on top of this we do add some stuff related to our applications. Let me write a simple Dockerfile and create a image from this

FROM debian
RUN apt update

* Now lets inspect the debian image docker image inspect debian and test image docker image inspect test:1.0

* The test:1.0 uses the layer from debian and adds a new layer this new layer is created as the result of the RUN statement (RUN apt update).
* Now lets build a new docker image test:2.0 with the following Dockerfile and inspect the image

FROM debian
RUN apt update
CMD ["echo", "helloworld"]

* Lets inspect test:2.0

* Lets update the Docker file and build the docker image test:3.0

FROM debian
RUN apt update
ADD test.txt /
CMD ["echo", "helloworld"]

* As we have observed so far, whenever we make changes that leads to some changes in the file system (RUN, ADD) a new layer is getting created

* Creating too many layers is not considered as good practice so you would often see Docker files with long RUN statements RUN statement1 && statement2

* The Layers get shared across images

* We would write Dockerfiles for our applications and create the docker images.

Docker image Registry

• We can store Docker images centrally in image Registries
  
• The Default Registries is DockerHub.
• There are many other Registries to Store docker images
• Cloud Registries:
  • Azure Container Registry
  • AWS ECR
  • Docker Hub (Public and Private)
  • And many other
• Hosted Registries
  • Artifactory/Jfrog
  • We can also use Docker Registry Image to store our images