docs: add bulding container docs

docs/build-headscale-container.md

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+# Build docker from scratch
+
+The Dockerfiles included in the repository are using the [buildx plugin](https://docs.docker.com/buildx/working-with-buildx/). This plugin is includes in docker newer than Docker-ce CLI 19.03.2. The plugin is used to be able to build different container arches. Building the Dockerfiles without buildx is not possible.
+
+# Build native
+
+To build the container on the native arch you can just use:
+```
+$ sudo docker buildx build -t headscale:custom-arch .
+```
+
+For example: This will build a amd64(x86_64) container if your hostsystem is amd64(x86_64). Or a arm64 container on a arm64 hostsystem (raspberry pi4).
+
+# Build cross platform
+
+To build a arm64 container on a amd64 hostsystem you could use:
+```
+$ sudo docker buildx build --platform linux/arm64 -t headscale:custom-arm64 .
+
+```
+
+**Import: Currently arm32 build are not supported as there is a problem with a library used by headscale. Hopefully this will be fixed soon.**
+
+# Build multiple arches
+
+To build multiple archres you could use:
+
+```
+$ sudo docker buildx create --use
+$ sudo docker buildx build --platform linux/amd64,linux/arm64 .
+
+```