Add crossbuilding

crossbuilding.md

+Cross Building
+==============
+
+## Raspbian (ARM)
+
+Since we want to support other platforms like Raspbian, I investigated an easy solution to run builds in our Docker
+build system.
+
+**Why not build on target platform?**
+
+Most ARM systems are pretty small sized since they are meant for IOT use, and don't need much computing power.
+
+On most Raspberry Pi a build can take multiple hours.
+
+**Why Docker?**
+
+It is well integrated in our build system and an easy solution for throw-away environments.
+
+### QEMU user
+
+The QEMU virtualization software can actually run builds in the same user space as the host, wrapping all CPU
+instructions during execution.
+
+Mainly this avoids running a complete VM for the foreign architecture, so any chroot or Docker build could benefit
+from a nearly native environment.
+
+```
+sudo apt install qemu-user-static
+qemu-user-static <any-arm-binary>
+```
+
+When inspecting the process list, you will see the foreign binary run with the interpreter, just
+as Python or Perl is visible.
+
+### Binfmt misc
+
+The Linux kernel has support for recognizing foreign binaries, and running them with a custom interpreter. A meta
+package configures the various interpreters installed.
+
+```
+sudo apt install binfmt-support
+sudo update-binfmts --display
+
+ls -al /proc/sys/fs/binfmt_misc/
+```
+
+### Using in Docker
+
+The environment inside Docker sees the same binfmt configuration as the host, and can simply use it.
+
+Only requirement is that the "interpreter" binary is also available inside the Docker container, this can be done via
+volume mounting:
+
+```
+docker run -it ... -v /usr/bin/qemu-arm-static:/usr/bin/qemu-arm-static:ro raspbian
+```
+
+For environments like GitLab this can be configured in the runner itself, so the volume is mounted every time:
+
+```toml
+[[runners]]
+  name = "docker-arm-static"
+  url = "https://git.icinga.com"
+  token = "xxx"
+  executor = "docker"
+  [runners.docker]
+    tls_verify = false
+    image = "alpine"
+    privileged = true
+    disable_entrypoint_overwrite = false
+    oom_kill_disable = false
+    disable_cache = false
+    volumes = ["/cache", "/usr/bin/qemu-arm-static:/usr/bin/qemu-arm-static:ro"]
+    shm_size = 0
+```
+
+### Kernel Support
+
+I noticed newer Kernels, at least 4.18 (I use Debian testing), supports this a bit better.
+
+This newer implementation does not seem to require the interpreter inside the container, but it seems to load it
+the interpeter binary from the host's filesystem namespace.
+
+Since it is a static binary, it doesn't need any additional resource.
+
+TODO: This is still under investigation.