[C#][aspnetcore] Fix broken Docker build option & documentation (#5128)

* Use lower-case packageName for README.md docker build example

* Use dotnet:1.0.3-sdk instead of just "latest"

* Regenerate Petstore server sample

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AspNetCoreServerCodegen.java

@@ -86,6 +86,7 @@ public class AspNetCoreServerCodegen extends AbstractCSharpCodegen {
         super.processOpts();
 
         additionalProperties.put("packageGuid", packageGuid);
+        additionalProperties.put("dockerTag", this.packageName.toLowerCase());
 
         apiPackage = packageName + ".Controllers";
         modelPackage = packageName + ".Models";