docs/devel: add some notes on the binfmt-image-debian targets

We document some of this on the wiki but lets move it into our
official developer notes documentation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-3-alex.bennee@linaro.org>

docs/devel/testing.rst

@@ -637,6 +637,44 @@ The full set of annotations can be found here:
 
 https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/tsan/rtl/tsan_interface_ann.cpp
 
+docker-binfmt-image-debian-% targets
+------------------------------------
+
+It is possible to combine Debian's bootstrap scripts with a configured
+``binfmt_misc`` to bootstrap a number of Debian's distros including
+experimental ports not yet supported by a released OS. This can
+simplify setting up a rootfs by using docker to contain the foreign
+rootfs rather than manually invoking chroot.
+
+Setting up ``binfmt_misc``
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can use the script ``qemu-binfmt-conf.sh`` to configure a QEMU
+user binary to automatically run binaries for the foreign
+architecture. While the scripts will try their best to work with
+dynamically linked QEMU's a statically linked one will present less
+potential complications when copying into the docker image. Modern
+kernels support the ``F`` (fix binary) flag which will open the QEMU
+executable on setup and avoids the need to find and re-open in the
+chroot environment. This is triggered with the ``--persistent`` flag.
+
+Example invocation
+~~~~~~~~~~~~~~~~~~
+
+For example to setup the HPPA ports builds of Debian::
+
+  make docker-binfmt-image-debian-sid-hppa \
+    DEB_TYPE=sid DEB_ARCH=hppa \
+    DEB_URL=http://ftp.ports.debian.org/debian-ports/ \
+    DEB_KEYRING=/usr/share/keyrings/debian-ports-archive-keyring.gpg \
+    EXECUTABLE=(pwd)/qemu-hppa V=1
+
+The ``DEB_`` variables are substitutions used by
+``debian-boostrap.pre`` which is called to do the initial debootstrap
+of the rootfs before it is copied into the container. The second stage
+is run as part of the build. The final image will be tagged as
+``qemu/debian-sid-hppa``.
+
 VM testing
 ----------