EntityType "Solution" is treated specially in these SearchContexts
|Solution "Delete Docker authentication token"
There might be an (apparently expired) authentication token in ~/.docker/config.json file, which the daemon then tried to use to pull the image from Dockerhub. Deleting the file resolved the problem.
|Solution "Download again and check $HASH"
Download the Composer installation script again and double check the value of the $HASH variable with echo $HASH. Once the installer is verified, you can continue with the next step.