add container-based development shell

It does not work with the graphical QEMU output but can build all
binaries.

container.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env sh
+
+# Run a shell inside of the container-based development environment.
+#
+# However, to see and use the QEMU UI, you need to run QEMU outside of the container.
+
+set -eu
+
+cd "$(dirname "$0")"
+
+if podman --version; then
+	cmd=podman
+else
+	cmd=docker
+fi
+
+"$cmd" build --tag=moslab .
+"$cmd" create --interactive --tty --name=moslab --volume="$(pwd)":"$(pwd)" \
+	--workdir="$(pwd)" moslab bash || true
+"$cmd" start --attach --interactive moslab