The original code works for me in a minimal example.
Lab is a room. A dog is in the Lab.
The wheelie bin is an enterable container in the Lab.
Instead of entering the wheelie bin:
move the player to the wheelie bin;
move the dog to the wheelie bin.
Have you tried running the code with “rules” and “actions” on in order to see exactly what’s happening? (That is, type “rules” and “actions” at the command prompt.)
For what it’s worth this also works with slightly different messages:
Carry out entering the wheelie bin:
now the dog is in the wheelie bin.
Though if you do something along the lines of Daniel’s suggestion, trying the actual action with the dog, you’ll get a more sensible default message about the dog entering the bin… which you probably will want to customize anyway.