Update install.rst

[conroydamien]

Adding PGRST_DB_ANON_ROLE: app_user to avoid the {"code":"PGRST302","details":null,"hint":null,"message":"Anonymous access is disabled"} and warning that it is not a good idea beyond just getting this docker-compose to work.

[wolfgangwalther]

Hm. I see the problem, aka "How to tell that I actually succeeded in setting this up via docker compose", but I don't really like the solution. It's seems quite arbitrary to set the anonymous role here, but not other settings which might very likely be relevant to actually work with the database. Although, I don't have a better solution either, right now.

[wolfgangwalther]

In general we need to make a better job on this page to explain:

• how to test whether the setup is successful
• how to continue configuration to get something actually usable

[wolfgangwalther]

I added my two observations to #3244 to keep track of those improvements we need.