Allow registry credentials for job/service containers (#694)

* Log in with container credentials if given

* Stub in registry aware auth for later

* Fix hang if password is empty

* Remove default param to fix build

* PR Feedback. Add some tests and fix parse

src/Sdk/DTPipelines/workflow-v1.0.json

@@ -373,7 +373,8 @@
           "options": "non-empty-string",
           "env": "container-env",
           "ports": "sequence-of-non-empty-string",
-          "volumes": "sequence-of-non-empty-string"
+          "volumes": "sequence-of-non-empty-string",
+          "credentials": "container-registry-credentials"
         }
       }
     },
@@ -404,6 +405,20 @@
       ]
     },
 
+    "container-registry-credentials": {
+      "context": [
+        "secrets",
+        "env",
+        "github"
+      ],
+      "mapping": {
+        "properties": {
+          "username": "non-empty-string",
+          "password": "non-empty-string"
+        }
+      }
+    },
+
     "container-env": {
       "mapping": {
         "loose-key-type": "non-empty-string",