Add config variables in order to create links outside of a request context

.env.tpl

@@ -31,10 +31,19 @@ HOST_DOCKER_GID=
 # Flask                                                                        #
 # https://flask.palletsprojects.com/en/1.1.x/config/                           #
 ################################################################################
+# CHOOSE ONE: http, https
+# DEFAULT: http
+# PREFERRED_URL_SCHEME=
+
 # DEFAULT: hard to guess string
 # HINT: Use this bash command `python -c "import uuid; print(uuid.uuid4().hex)"`
 # SECRET_KEY=
 
+# Example: nopaque.example.com nopaque.example.com:5000
+# HINT: If your instance is publicly available on a different Port then 80/443,
+#       you will have to add this to the server name
+SERVER_NAME=
+
 # CHOOSE ONE: False, True
 # DEFAULT: False
 # HINT: Set to true if you redirect http to https