Update building_images.rst

Adding label to be able to reference the documentation section

content/building_images.rst

@@ -298,6 +298,7 @@ For more info on how to set environment variables when running containers,
 see `this page <https://apptainer.org/docs/user/latest/environment_and_metadata.html>`__
 from Apptainer's documentation.
 
+.. _documentation:
 
 Adding documentation to your image
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