Rework container creation

- simplify the process by having the riddler container build the rootfs and config
- output tarred up rootfs and config.json as otherwise file ownership not preserved
- allow easy build of a collection of container tarballs with another conversion script

This makes it easy to choose which container images you want and just convert any
set to a initrd image
```
tar cf - container1.tar container2.tar | docker run -i tartar2initrd > initrd.img
```

Next stage will use a manifest to select the ones to add for each edition.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

base/riddler/Dockerfile

@@ -9,6 +9,7 @@ RUN \
   jq \
   linux-headers \
   musl-dev \
+  tar \
   && true
 
 COPY Dockerfile /