Thanks @pfaffman! Sorry about the confusion, I mixed up two things in my original message: 1) installing Discourse for the Gitpod community, and 2) automating the dev setup for Discourse developers and contributors. I’m thankfully following the developer guide for 2), not for 1). 
@david Hmm, unfortunately, with:
bundle exec rake db:create db:migrate &&
RAILS_ENV=test LOAD_PLUGINS=1 bundle exec rake db:create db:migrate
It still fails with:
== Seed from /workspace/discourse/db/fixtures/990_settings.rb
Discourse hostname: localhost is not a valid domain for emails!
== Seed from /workspace/discourse/db/fixtures/990_topics.rb
rake aborted!
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: relation "polls" does not exist
LINE 8: WHERE a.attrelid = '"polls"'::regclass
^
Did I understand your suggestion correctly? Would you have other ideas on how to fix it? (I tried to drop the tables beforehand, but that failed with a different error which I don’t remember. Happy to try again though.)
Also, I forgot to mention that my setup instructions worked until last week or so, so the error appeared recently after a Discourse rebase. I think after the migration to Rails 6 maybe? https://github.com/discourse/discourse/commit/32b8a2ccffd50d78b8f7be6f0b524506cd2dbd7e
FYI, you can easily reproduce the error by trying to open my Discourse fork in Gitpod: Gitpod - Dashboard . You’ll see my automated setup fail, and get an interactive environment where one can try various Discourse commands in the Terminal.