Also keep track of directory creation there, so you can explicitly
set directory permissions if required, and to avoid duplicates.
We should really keep track of files created elsewhere in the build
as well as we still might create some extras, but at least you can
set the write permisisons.
We can add uid, gid support too if required...
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
This removes outputs from yaml, instead you can do
```
moby build -output tar -output qcow2 file.yaml
```
or alternative syntax
```
moby build -output tar,qcow2 file.yaml
```
In future we may change this to be available in a `moby package`
step, but lets try this for now.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
