Common usage with Docker

• What is a Dockerfile?
  A Dockerfile is a high-level script containing instructions for building a Docker image.

• What is a Docker image?
  The Docker image is a snapshot of the environment, including the base OS, dependencies, and app files.

  It is an instruction set of how Linux namespaces and c-groups should be configured.

• How does it work?

  1. When docker build runs (ie. Docker CLI), Docker serializes the Docker file into a layered, immutable image, which in simpler terms means its an instruction set how the daemon should create the app within the namespace and c-groups in Linux. For example if its a C# app - fetching the ASP.NET runtime and .NET SDK. Otherwise, it cannot compile or even run the .dll. So the the Docker file only contains higher level instructions to restore (nuget packages) and compile a C# project if we are talking about a C# project, but it could be anything.

  2. The Docker Daemon (dockerd) reads the image and uses Linux kernel features to isolate and manage the container:

     • Namespaces: Ensure process, network, and filesystem isolation. So Namespaces is a feature of the Linux kernel that partitions kernel resources such that one set of processes sees one set of resources while another set of processes sees a different set of resources.

     • Cgroups: Limit resource usage (CPU, memory, etc.).

• What happens when the container runs?
  The container runs the application in an isolated environment as defined by the CMD or ENTRYPOINT in the Dockerfile.

• Quick Analogy:

  • The Dockerfile is like a recipe.
  • The Docker image is the prepared dish, ready to serve.
  • The Docker Daemon (dockerd) is the chef who reads the recipe, prepares the dish, and serves it.

• Architecture: Layer 0 - Hardware Layer 1 - BIOS Layer 2 - Hypervisor (either HyperV (Win), HyperKit (macOS)) Layer 3 - A virtual machine that runs a lightweight Linux instance. This is where the docker daemon lives so it can use the Linux features of Namespaces and C-groups, which makes the actual "Container"!