[WEBSITE] Point the docker release to our own Hub repository
Summary:
We now have our own repository on Docker Hub for hosting the images, so
use it.
Test Plan:
make serve
Navigate to the release page and click the docker button. Check the link
points to our own repo.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7843