new "image" mount type

Add a new "image" mount type to `--mount`.  The source of the mount is
the name or ID of an image.  The destination is the path inside the
container.  Image mounts further support an optional `rw,readwrite`
parameter which if set to "true" will yield the mount writable inside
the container.  Note that no changes are propagated to the image mount
on the host (which in any case is read only).

Mounts are overlay mounts.  To support read-only overlay mounts, vendor
a non-release version of Buildah.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/fsouza/go-dockerclient/container_prune.go

@@ -0,0 +1,40 @@
+package docker
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+)
+
+// PruneContainersOptions specify parameters to the PruneContainers function.
+//
+// See https://goo.gl/wnkgDT for more details.
+type PruneContainersOptions struct {
+	Filters map[string][]string
+	Context context.Context
+}
+
+// PruneContainersResults specify results from the PruneContainers function.
+//
+// See https://goo.gl/wnkgDT for more details.
+type PruneContainersResults struct {
+	ContainersDeleted []string
+	SpaceReclaimed    int64
+}
+
+// PruneContainers deletes containers which are stopped.
+//
+// See https://goo.gl/wnkgDT for more details.
+func (c *Client) PruneContainers(opts PruneContainersOptions) (*PruneContainersResults, error) {
+	path := "/containers/prune?" + queryString(opts)
+	resp, err := c.do(http.MethodPost, path, doOptions{context: opts.Context})
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	var results PruneContainersResults
+	if err := json.NewDecoder(resp.Body).Decode(&results); err != nil {
+		return nil, err
+	}
+	return &results, nil
+}