Add instructions on how to use the packaging under other distros using podman

These instructions are still pretty raw. Creating helper scripts to make
the use easier would make sense at some point.

README.md

@@ -109,6 +109,56 @@ is removed in the end. Set `DEBUG=1` to prevent that. Then one can use e.g.
 `podman container start …` and `podman container exec -it … bash` to enter the
 container for manual investigation.
 
+### Using Arch-packages on another distribution via a container
+If you want to cross-compile software on non-Arch distributions you can make use
+of the `android-*` and `mingw-w64-*` packages provided by this repository using
+an Arch Linux container. The container image mentioned before is also suitable
+for this purpose.
+
+Here are some example commands how one might do that:
+```
+# create a directory to store builds and new container and start it
+mkdir -p /hdd/build/container
+podman container create -it \
+  --name archlinux-devel-container \                    # give container a meaningful name
+  -v /hdd/cache/pacman/pkg:/var/cache/pacman/pkg \      # share pacman cache accross different containers
+  -v /hdd/build/container:/build \                      # expose build directory to host
+  -v /hdd/projects:/src \                               # access source files from host
+  -v /hdd/chroot/remote-config-x86_64:/cfg \            # mount directory containing pacman.conf/makepkg.conf
+  archlinux-base-devel
+podman container start archlinux-devel-container
+
+# configure pacman to use config from mounted directory
+podman container exec archlinux-devel-container bash -c "$(cat devel/container/containersync)"
+
+# start interactive shell in container
+podman container exec -it archlinux-devel-container bash
+
+# install stuff you want
+podman container exec -it archlinux-devel-container \
+  pacman -Syu ninja git mingw-w64-cmake qt6-{base,tools} mingw-w64-qt6-{base,tools,translations,svg,5compat}
+
+# configure the build, e.g. run CMake
+podman container exec -it archlinux-devel-container x86_64-w64-mingw32-cmake \
+  -G Ninja \
+  -S /src/c++/cmake/PianoBooster \
+  -B /build/pianobooster-x86_64-w64-mingw32-release \
+  -DQT_PACKAGE_NAME:STRING=Qt6
+
+# conduct the build, e.g. invoke Ninja build system via CMake
+podman container exec -it archlinux-devel-container cmake \
+  --build /build/pianobooster-x86_64-w64-mingw32-release
+
+# get rid of the container when no longer needed
+podman container stop archlinux-devel-container
+podman container rm archlinux-devel-container
+```
+
+Note that these commands are intended to be run without root (see section
+"Podman-specific remarks" for details). In this case files that are created
+from within the container in the build and source directories will have your
+normal user/group outside the container which is quite convenient (within the
+container they will be owned by root).
 
 ### Other approaches
 There's also the 3rdparty repository

devel/container/containersync

@@ -3,5 +3,5 @@ set -e
 for config_file in {/cfg/,}{makepkg,pacman}.conf; do
     [[ -f $config_file ]] && cp --target-directory=/etc "$config_file"
 done
-source PKGBUILD
+[[ -f PKGBUILD ]] && source PKGBUILD
 pacman -Syu --noconfirm --needed "${depends[@]}" "${makedepends[@]}" "${checkdepends[@]}"