Use github.com/containers/image to get image digests

It appear the way we were looking for image digests might not work with
some registries because of different docker registry references used.
Also getting the digest with signed registries is quite complex.
So use github.com/containers/image which provide the proper way for
working with registries.

We lost auto-detection of imagePullSecrets, but I think it's ok to let
the user mount the secret to have a ~/.docker/config.json (so RBAC
doesn't need to have access to Secrets).

Closes #13

README.md

@@ -117,7 +117,6 @@ objects.
 
 ## Docker credentials
 
-By default imago will looks for docker registry credentials in:
-
-* imagePullSecrets in the pod template specification
-* ~/.docker/config.json (e.g. /var/lib/imago/.docker/config.json in docker image)
+Image will looks for docker registry credentials in ~/.docker/config.json (e.g.
+/var/lib/imago/.docker/config.json in docker image).
+So, in case you're using `imagePullSecrets`, you will have to mount the secret here.

deploy/serviceaccount.yaml

@@ -28,12 +28,6 @@ rules:
     - get
     - list
     - update
-  - apiGroups:
-      - ""
-    resources:
-    - secrets
-    verbs:
-    - get
   - apiGroups:
       - ""
       - apps

go.mod

@@ -1,9 +1,7 @@
 module github.com/philpep/imago
 
 require (
-	github.com/stretchr/objx v0.2.0 // indirect
-	github.com/stretchr/testify v1.4.0
-	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect
+	github.com/containers/image/v5 v5.4.3
 	k8s.io/api v0.0.0-20191016110408-35e52d86657a
 	k8s.io/apimachinery v0.0.0-20191004115801-a2eda9f80ab8
 	k8s.io/client-go v0.0.0-20191016111102-bec269661e48