CI: add runtime tests for packages

Additional to manual runtime tests this CI addition runs a custom test
script per package. Ideally this lowers the errors of package bumps,
something which is time consuming when done manually for multiple
architectures.

This CI uses the official OpenWrt containers and tries to install and
run compiled packages. The run depends on the content of `test.sh`,
which is an `ash` script. It's called with the *packge name* and
*package version* as arguments. This allows different behaviour if
a single package generates multiple IPK files. The version is usable for
the most trivial runtime check, e.g. `tmux -V | grep "$2"`.

The current approach uses the qus project[1] which contains multiple
QEMU binaries to run various architectures.

[1]: https://github.com/dbhi/qus

Signed-off-by: Paul Spooren <mail@aparcar.org>

.github/workflows/Dockerfile

@@ -0,0 +1,6 @@
+ARG ARCH=x86-64
+FROM openwrtorg/rootfs:$ARCH
+
+ADD entrypoint.sh /entrypoint.sh
+
+CMD ["/entrypoint.sh"]