Allow ASL_RULEBOOK2_BASE_URL to be set from an env.var.

vasl_templates/webapp/__init__.py

@@ -230,6 +230,7 @@ _set_config_from_env( "VASL_EXTNS_DIR" )
 _set_config_from_env( "BOARDS_DIR" )
 _set_config_from_env( "CHAPTER_H_NOTES_DIR" )
 _set_config_from_env( "USER_FILES_DIR" )
+_set_config_from_env( "ASL_RULEBOOK2_BASE_URL" )
 # NOTE: The Docker container also sets DEFAULT_TEMPLATE_PACK, but we read it directly from
 # the environment variable, since it is not something that is stored in app.config.