docs: contributing docs update for cross comp

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/contributing/README.md

@@ -6,5 +6,6 @@
  * (Optional) [Configure project for IDE](set-up-ide.md)
  * [Configure Git for contributing](set-up-git.md)
  * [Work with a development container](set-up-dev-env.md)
+ * [Containerized build and cross compilation](ctn-build.md)
  * [Run tests and test documentation](test.md)
  * [Debugging the daemon](debug.md)

docs/contributing/ctn-build.md

@@ -0,0 +1,35 @@
+The `Dockerfile` supports building and cross compiling docker daemon and extra
+tools using [Docker Buildx](https://github.com/docker/buildx) and [BuildKit](https://github.com/moby/buildkit).
+A [bake definition](https://github.com/docker/buildx/blob/master/docs/reference/buildx_bake.md)
+named `docker-bake.hcl` is in place to ease the build process:
+
+```shell
+# build binaries for the current host platform
+# output to ./bundles/binary by default
+docker buildx bake
+
+# build binaries for the current host platform
+# output to ./bin
+DESTDIR=./bin docker buildx bake
+
+# build dynamically linked binaries
+# output to ./bundles/dynbinary by default
+DOCKER_LINKMODE=dynamic docker buildx bake
+
+# build binaries for all supported platforms
+docker buildx bake binary-cross
+
+# build binaries for a specific platform
+docker buildx bake --set *.platform=linux/arm64
+
+# build all for the current host platform (binaries + containerd, runc, tini, ...)
+# output to ./bundles/all by default
+docker buildx bake all
+
+# build all for the current host platform
+# output to ./bin
+DESTDIR=./bin docker buildx bake all
+
+# build all for all supported platforms
+docker buildx bake all-cross
+```