• Docker is a containerization platform which packages your application and all its dependencies together in the form of containers so as to ensure that your application works seamlessly in any environment be it development or test or production.
• Docker containers, wrap a piece of software in a complete filesystem that contains everything needed to run: code, runtime, system tools, system libraries etc. anything that can be installed on a server.
• This guarantees that the software will always run the same, regardless of its environment.

An instance of an image is called a container. You have an image, which is a set of layers. If you start this image, you have a running container of this image. You can have many running containers of the same image.

You can see all your images with docker images whereas you can see your running containers with docker ps (and you can see all containers with docker ps -a).

So a running instance of an image is a container.

Although ADD and COPY are functionally similar, generally speaking, COPY is preferred.

That’s because it’s more transparent than ADD. COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious. Consequently, the best use for ADD is local tar file auto-extraction into the image, as in ADD rootfs.tar.xz /.

Docker hub is a cloud-based registry service which allows you to link to code repositories, build your images and test them, stores manually pushed images, and links to Docker cloud so you can deploy images to your hosts. It provides a centralized resource for container image discovery, distribution and change management, user and team collaboration, and workflow automation throughout the development pipeline.

There are four states that a Docker container can be in, at any given point in time. Those states are as given as follows:

• Running
• Paused
• Restarting
• Exited

If you must stop the container really quickly… (someone pushed something to production on Friday evening?… ;) )

We can identify the status of a Docker container by running the command

docker ps –a

which will in turn list down all the available docker containers with its corresponding statuses on the host. From there we can easily identify the container of interest to check its status correspondingly.

The primary difference is that using ‘docker create’ creates a container in a stopped state.

Bonus point: You can use ‘docker create’ and store an outputed container ID for later use. The best way to do it is to use ‘docker run’ with --cidfile FILE_NAME as running it again won’t allow to overwrite the file. A good practice is to keep well ogranised directory structure: /containers/web/server1/ws.cid containers/web/server3/ws.cid

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
2. ENTRYPOINT should be defined when using the container as an executable.
3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
4. CMD will be overridden when running the container with alternative argumen

• Docker registry is a service for hosting and distributing images (the default one is the Docker Hub).
• Docker repository is a collection of related Docker images (the same name but with different tags).

There is no loss of data when any of your Docker containers exits as any of the data that your application writes to the disk in order to preserve it. This will be done until the container is explicitly deleted. The file system for the Docker container persists even after the Docker container is halted.

No, to remove a container it must be stopped first.

When we build an Image, Docker will process each line in Dockerfile. It will execute the commands on each line in the order that is mentioned in the file. But at each line, before running any command, Docker will check if there is already an existing image in its cache that can be reused rather than creating a new image.

There are three main features helping to achieve that:

• Volumes
• Environment variable injection
• Read-only file systems

The simplest way is to use network port mapping. There’s also the - -link flag which is deprecated.