Improve build docs

This commit eliminates redundancies between README.md and BUILD.md. Not having to maintain redundant documentation is a big improvement as there is no chance they will be out of sync. Plus, the docs in BUILD.md are a bit more sophisticated while still allowing @probonopd to copy-and-paste commands for his workflow.

Additionally, the environment variable used in the chroot scripts has been fixed.

The commit further adds a short introduction explaining the differences between the build scripts and recommending Docker for regular users.

Author: TheAssassin

BUILD.md

@@ -1,5 +1,32 @@
 # How to build the runtime
 
+We maintain and provide two official ways to build the runtime:
+
+- a Docker-based setup that caches dependencies and isolates the build environment from the system
+- a `chroot`-based method that can be used in isolated environments like, e.g., GitHub codespaces, if you cannot use Docker
+
+**Please note: We recommend regular users to use the Docker-based setup whenever possible!** The chroot based setup imposes a risk to break your local machine. It is meant **only** for environments that are otherwise isolated or reproducible, e.g., GitHub codespaces. 
+
+
+## chroot-based environment
+
+The chroot-based environment is designed for people who really do not want to use containers and/or run on systems that do not support such an environment (e.g., GitHub codespaces, FreeBSD).
+
+To run a build, use the following command:
+
+```sh
+> env ALPINE_ARCH=<arch>  chroot/chroot_build.sh
+
+# example calls:
+> env ALPINE_ARCH=x86_64  chroot/chroot_build.sh
+> env ALPINE_ARCH=i686    chroot/chroot_build.sh
+> env ALPINE_ARCH=armhf   chroot/chroot_build.sh
+> env ALPINE_ARCH=aarch64 chroot/chroot_build.sh
+```
+
+The script will download an Alpine miniroot image, extract it into a specific location, bind-mount a set of temporary filesystems (e.g., `/proc`) there, chroot into there and run the build script. It attempts to unmount the previously mounted paths again.
+
+
 ## Docker
 
 As the runtime build requires a special environment with specific dependencies prebuilt and installed as static binaries, we provide a containerized build environment. We use Docker as a runtime for now.
@@ -65,12 +92,3 @@ To specify commands that should be run, use the established `--` to distinguish
 > env ARCH=<arch> scripts/create-build-container.sh -u $(id -u):(id -g) -- bash some-script.sh
 ```
 
-## chroot-based environment
-
-The chroot-based environment is designed for people who really do not want to use containers and/or run on systems that do not support such an environment (e.g., FreeBSD).
-
-To run a build, use the following command:
-
-```sh
-> env ARCHITECTURE=<arch> chroot/chroot_build.sh
-```

README.md

@@ -17,16 +17,9 @@ __Please note:__ This repository is meant to be extremely simple.
 * Do NOT add additional external dependencies or files. Everything shall be implemented in one file. `runtime.c`  
 * Do NOT add a complicated "build system" (like autotools, CMake,...) other than the existing simple Makefile and bash
 
-Binaries are provided on GitHub Releases. Should you need to build the runtime locally or on GitHub Codespaces, the following will build the contents of this repository in an Alpine container:
+Binaries are provided on GitHub Releases. 
 
-```
-export ALPINE_ARCH=x86_64
-./scripts/chroot/chroot_build.sh # Or execute the steps in it manually
-```
-
-This whole process takes only a few seconds, e.g., on GitHub Codespaces.
-
-See [BUILD.md](BUILD.md) for more information, including on how to build using Docker.
+**Please see [BUILD.md](BUILD.md) if you want to build the runtime yourself.**
 
 ## Signing