They way I see it there are two ways this note can be resolved:
1. The container is temporary and should not be mapped as a building. Remove it from OSM.
2. The container is used as a permanent building (even if it can technically be moved) and we map it as building=container.

From your description that it's been there for years and is used as a shed, option 2 seems most correct to me. Maybe you can have a look at the wiki page for building and see if you agree with me on my interpretation of building=container? https://wiki.openstreetmap.org/wiki/Key:building#Other_buildings