added readme for building the docker containers

docker/README.md

@@ -0,0 +1,25 @@
+# Docker Containers for c3nav
+
+
+## Requirements for building
+
+You need docker buildx and qemu-user-static and a buildkit builder that is using the docker-container or kubernetes
+driver if you want to build multi-arch images.
+
+This are the necessary steps to get it working on arch linux
+
+```bash
+pacman -Sy docker-buildx qemu-user-static
+docker buildx create --driver=docker-container --bootstrap --use
+```
+
+Additonally you need to be signed in into the github container registry. A guid for how to do this can be found 
+[here](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-with-a-personal-access-token-classic).
+
+
+## Building
+
+You can run the `build.sh` script in two modes. If you run it without any arguments it uses your local git tree
+including any uncommitted changes to build the docker containers.
+
+If you run `./build.sh git` it will do a fresh git checkout of the same commit as you currently on for building.