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/debian.fs.do

@@ -0,0 +1,24 @@
+exec >&2
+fs=${1%.fs}
+rm -rf "$fs" "$fs.fakeroot"
+
+redo-ifchange debootstrap.fs
+fakeroot -i debootstrap.fakeroot -s "$fs.fakeroot" cp -a debootstrap/. "$fs"
+
+# Work around bug (in fakechroot?) where /lib64 symlink ends up pointing
+# at an absolute path including $PWD, rather than inside the chroot.
+# Rather than fix the symlink, we'll just make sure $PWD is a link to /,
+# so that the "wrong" symlinks correctly resolve.
+pwdir=$(dirname "$PWD/bootstrap/")
+mkdir -p "$fs/$pwdir/debootstrap"
+dots=$(echo "$pwdir/" | sed -e 's,[^/]*/,../,g')
+ln -s "${dots}lib" "$fs/$pwdir/debootstrap/lib"
+
+# /init script is what we run in 'docker run'
+cat >"$fs/init" <<-EOF
+	#!/bin/sh
+	dpkg -l | wc -l
+EOF
+chmod a+x "$fs/init"
+
+redo-ifchange "$fs/bin/sh"