@ajaypalnitj I also have the same or a similar issue on two relatively new installs. No issue for the other older installs though. Upgrade page is a blank white page.
Edit: Can confirm that docker_manager is installed (default). GitHub - discourse/docker_manager: plugin for use with discourse docker image
Update: Seems to be fixed now without any action on my part. 
Wasn’t working ~30 mins ago and for the days prior.
There were some recent bugs in the updater, you will need to upgrade at the command line.
Thanks a lot.
Do I install it like other plugins only?
I checked my app.yml and the docker manager is there. It was not showing in my admin->plugins list. But still not working.
Just rebuild ./launcher rebuild app
That will fix everything
I used rebuild to update the website. But that still doesn’t show upgrade option on website.
Can you please share a screenshot of your app.yml 's plugin section?
Also, do you know which version of Discourse you are using? It’s in the app.yml (tests-passed, beta, stable).