"ExternalRestoreSources" needs to be set in the docker container (#8602)

* Add "PB_RESTORESOURCE" to docker environmental variables.

* Add "PB_RestoreSource" explictly to the 'SandBox' build.sh call.

* Add "ExternalRestoreSources" explictly to the 'SandBox' build.sh call.

* Using an environmental variable to communicate to the docker container.

Author: johnbeisner

scripts/dockerrun.sh

@@ -139,5 +139,6 @@ docker run $INTERACTIVE -t --rm --sig-proxy=true \
     -e PB_ASSETROOTURL \
     -e PB_PACKAGEVERSIONPROPSURL \
     -e PB_PUBLISHBLOBFEEDURL \
+    -e EXTERNALRESTORESOURCES \
     $DOTNET_BUILD_CONTAINER_TAG \
     $BUILD_COMMAND "$@"