Replace riddler with code that constructs config.json directly

Generated largely from the specified config; small parts taken from `docker image inspect`,
such as the command line.

Renamed some of the yaml keys to match the OCI spec rather than Docker Compose as
we decided they are more readable, no more underscores.

Add some extra functionality
- tmpfs specification
- fully general mount specification
- no new privileges can be specified now

For nostalgic reasons, using engine-api to talk to the docker cli as
we only need an old API version, and it is nice and easy to vendor...

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

vendor/github.com/docker/engine-api/client/image_save.go

@@ -0,0 +1,22 @@
+package client
+
+import (
+	"io"
+	"net/url"
+
+	"golang.org/x/net/context"
+)
+
+// ImageSave retrieves one or more images from the docker host as an io.ReadCloser.
+// It's up to the caller to store the images and close the stream.
+func (cli *Client) ImageSave(ctx context.Context, imageIDs []string) (io.ReadCloser, error) {
+	query := url.Values{
+		"names": imageIDs,
+	}
+
+	resp, err := cli.get(ctx, "/images/get", query, nil)
+	if err != nil {
+		return nil, err
+	}
+	return resp.body, nil
+}