I fixed that by running `export SECRET_KEY_BASE=xxxxxx` and `export OTP_SECRET=xxxxxx` (values from the prior rake secret calls), and then I could call `rake mastodon:webpush:generate_vapid_key` and get valid values. I'm working through the linked process, but trying to use the all-in-one docker image from https://github.com/Wonderfall/docker-mastodon. Note that you still have to run the non-Mastodon services, like Postgres and Redis, separately.