Allow the container.image to be empty (#2235)

* Allow container.image to be null/empty

* Remove unnecessary workflow type

* Update src/Sdk/DTPipelines/Pipelines/ObjectTemplating/PipelineTemplateConverter.cs

Co-authored-by: Ferenc Hammerl <31069338+fhammerl@users.noreply.github.com>

* Rename ConvertToJobContainer to ConvertToContainer
It converts serviceContainers too

* Remove unused parameter
AllowExpressions was always false

* Allow service in yaml with empty string for value

* Don't attempt to start services without an image

* Improve error messages

* Revert "Remove unused parameter"

This reverts commit f4ab2d50d0.

* Revert "Rename ConvertToJobContainer to ConvertToContainer"

This reverts commit ffc62f8343.

* Fix revert merge conflict

* Removed info logs

* Added info log for services without container images

Co-authored-by: moonblade <moonblade168@gmail.com>
Co-authored-by: Ferenc Hammerl <31069338+fhammerl@users.noreply.github.com>
Co-authored-by: Ava Stancu <avastancu@github.com>

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

@@ -381,7 +381,7 @@
     "container-mapping": {
       "mapping": {
         "properties": {
-          "image": "non-empty-string",
+          "image": "string",
           "options": "non-empty-string",
           "env": "container-env",
           "ports": "sequence-of-non-empty-string",
@@ -414,7 +414,7 @@
         "vars"
       ],
       "one-of": [
-        "non-empty-string",
+        "string",
         "container-mapping"
       ]
     },