I ran into this problem yesterday upgrading from beta7 to beta8; got my dev to run the ./launcher rebuild command and it seems to have resolved. Interested to see the solution/possible cause.
For me it was ‘update docker manager’ first, then ‘update all’ - which did 4 plugins and discourse itself. When it came back up, it showed that discourse and one of the plugins still had an update to run. Clicking update all again just brought up the terminal, and black for over an hour. Refreshing the page brought on the ‘old version of discourse image’.