Merge branch 'doc/README' into 'master'

Make README more user-centric and explain CI processes

Closes #40 and #48

See merge request !51

README.md

 # Arch Linux Docker Image
+
 [![pipeline status](https://gitlab.archlinux.org/archlinux/archlinux-docker/badges/master/pipeline.svg)](https://gitlab.archlinux.org/archlinux/archlinux-docker/-/commits/master)
 
-This repository contains all scripts and files needed to create a Docker image for Arch Linux.
+Arch Linux provides Docker images both in the [official DockerHub library](https://hub.docker.com/_/archlinux) (`docker pull library/archlinux:latest`) and in our [own repository](https://hub.docker.com/r/archlinux/archlinux) (`docker pull archlinux/archlinux:latest`).
+
+Images in the official library are updated weekly while our own repository is updated daily.
+
+Two versions of the image are provided: `base` (~160MB) and `base-devel` (~240MB), containing the respective meta package / package group. Both are available as tags with `latest` pointing to `base`. Additionally, images are tagged with their date and build job number, f.e. `base-devel-20201118.0.9436`.
+
+While the images are regularly kept up to date it is strongly recommended running `pacman -Syu` right after starting a container due to the rolling release nature of Arch Linux.
+
+## Principles
+* Provide the Arch experience in a Docker image
+* Provide the simplest but complete image to `base` and `base-devel` on a regular basis
+* `pacman` needs to work out of the box
+* All installed packages have to be kept unmodified
+
+## Building your own image
+
+[This repository](https://gitlab.archlinux.org/archlinux/archlinux-docker) contains all scripts and files needed to create a Docker image for Arch Linux.
 
-## Dependencies
+### Dependencies
 Install the following Arch Linux packages:
 
 * make
@@ -12,15 +29,23 @@ Install the following Arch Linux packages:
 * fakechroot
 * fakeroot
 
-Make sure your user can directly interact with Docker (ie. `docker info` works).
+Make sure your user can directly interact with Docker (i.e. `docker info` works).
 
-## Usage
-Run `make docker-image-base` to build the image `archlinux/archlinux:base` with the
-`base` group installed. You can also run `make docker-image-base-devel` to
-build the image `archlinux/archlinux:base-devel` with the `base-devel` group installed.
+### Usage
+Run `make docker-image-base` to build the `archlinux:base` image with the
+`base` meta package installed. You can also run `make docker-image-base-devel` to
+build the image `archlinux:base-devel` which additionally has the `base-devel` group installed.
 
-## Principles
-* Provide the Arch experience in a Docker image
-* Provide the most simple but complete image to base every other upon
-* `pacman` needs to work out of the box
-* All installed packages have to be kept unmodified
+## Pipeline
+
+### Daily builds
+
+Daily images are build with scheduled [GitLabCI](https://gitlab.archlinux.org/archlinux/archlinux-docker/-/blob/master/.gitlab-ci.yml) using our own runner infrastructure. Initially root filesystem archives are constructed and provided in our [package registry](https://gitlab.archlinux.org/archlinux/archlinux-docker/-/packages). The released multi-stage Dockerfile downloads those archives and verifies their integrity before unpacking it into a Docker image layer. Images are build using [kaniko](https://github.com/GoogleContainerTools/kaniko) to avoid using privileged Docker containers, which also publishes them to our DockerHub repository.
+
+### Weekly releases
+
+Weekly releases to the official DockerHub library use the same pipeline as daily builds. Updates are provided as automatic [pull requests](https://github.com/docker-library/official-images/pulls?q=is%3Apr+archlinux+is%3Aclosed+author%3Aarchlinux-github) to the [official-images library](https://github.com/docker-library/official-images/blob/master/library/archlinux), whose GitHub pipeline will build the images using our provided rootfs archives and Dockerfiles.
+
+### Development
+
+Changes in Git feature branches are build and tested using the pipeline as well. Development images are uploaded to our [GitLab Container Registry](https://gitlab.archlinux.org/archlinux/archlinux-docker/container_registry).
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