docs: improve documentation on admin secret ☂️

[marionschleifer]

We need to improve documentation on admin secret.

The following points need to be covered:

• Document restrictions on admin-secret #2116 - Can we add the maximum length / character set for admin-secret to the README?
  (e.g. as AWS does)
• docs: add top level entry on configuring hasura CLI with admin secret #1725
• Explain what admin secret is and does: https://docs.hasura.io/1.0/graphql/manual/deployment/docker/securing-graphql-endpoint.html

[tbenst]

@marionschleifer did something change from 1.0.0 -> 1.2.2 that affects how to pass admin-secret?

I'm seeing same issue as #4926 from the following commands:

docker run -p 8080:8080 --rm \
       -e HASURA_GRAPHQL_DATABASE_URL=postgres://$POSTGRES_USER:$POSTGRES_PASSWORD@$POSTGRES_SERVER:$POSTGRES_PORT/$DBNAME \
       -e HASURA_GRAPHQL_ADMIN_SECRET=$HASURA_GRAPHQL_ADMIN_SECRET \
       hasura/graphql-engine:v1.2.2 &

hasura-cli console \
    --admin-secret $HASURA_GRAPHQL_ADMIN_SECRET

[scriptonist]

@tbenst Thanks for the feedback, Nothing changed on how the admin secret is passed, but this may be a bug, will look into it.

[tbenst]

@scriptonist for me, the issue was I was running these commands on a remote server, and port forwarding only the console port (9695). I'm guessing that browser-side, the console attempts to connect to localhost:8080. When I ran console locally, it worked fine.

To reproduce, ssh to a remote computer, forwarding whatever port you use for hasura-cli run the two commands in #4055 (comment) on remote. On local, go to localhost:9695, and see the invalid admin-secret error.