Add troubleshooting for "only updates to Labels are allowed"

docs/abra/trouble.md

@@ -101,3 +101,7 @@ Otherwise, you can try manually cloning the recipe repository to the correct loc
 ```
 $ git clone https://git.coopcloud.tech/coop-cloud/MyCoolRecipe.git ~/.abra/recipes
 ```
+
+## "only updates to Labels are allowed"
+
+See [Packaging handbook » What does "only updates to Labels are allowed" mean](maintainers/handbook/#what-does-only-updates-to-labels-are-allowed-mean).

docs/maintainers/handbook.md

@@ -386,6 +386,19 @@ It is good practice to take note of all the issues you ran into and share them w
 
 If you don't have time or are not an operator, reach out on our communication channels for an operator willing to do some testing.
 
+## What does "only updates to Labels are allowed" mean
+
+If you see something like this:
+
+```
+FATA failed to update config traefik_traefik_yml_v22: Error response from daemon: rpc error: code = InvalidArgument desc = only updates to Labels are allowed
+```
+
+It means that a Docker "config" has been updated, but the version number has not been incremented.
+
+To fix this, edit a recipe's `abra.sh` and update the version number of the relevant line –­in this case, `export TRAEFIK_YML_VERSION=v22`.
+`
+
 ## How do I write version release notes?
 
 In the root of your recipe repository, run the following (if the folder doesn't already exist):