cookbook/container: example of building+running docker containers.

This got... long... and complicated.  But I think it's a really good
demonstration of getting redo to do complicated things elegantly.  At
least, I hope it is.

docs/cookbook/container/debootstrap.fs.do

@@ -0,0 +1,19 @@
+exec >&2
+fs=${1%.fs}
+rm -rf "$fs" "$fs.fakeroot"
+
+redo-ifchange debdownload.fs debootstrap.options
+cp -a debdownload/. "$fs"
+eatmydata \
+	fakechroot \
+	fakeroot -s "$fs.fakeroot" \
+	debootstrap $(cat debootstrap.options) "$fs"
+
+# Clean up installed package files
+rm -f "$fs"/var/cache/apt/archives/*.deb \
+	"$fs"/var/cache/apt/*.bin \
+	"$fs"/var/lib/apt/lists/*Packages \
+	"$fs"/var/lib/apt/lists/*Sources \
+	"$fs"/var/lib/apt/lists/debootstrap*
+
+redo-ifchange "$fs/bin/sh"