docs: update contributing documentation (toeverything#8486)

.devcontainer/build.sh

@@ -13,6 +13,3 @@ yarn workspace @affine/server-native build
 
 # Create database
 yarn workspace @affine/server prisma db push
-
-# Create user username: affine, password: affine
-echo "INSERT INTO \"users\"(\"id\",\"name\",\"email\",\"email_verified\",\"created_at\",\"password\") VALUES('99f3ad04-7c9b-441e-a6db-79f73aa64db9','affine','[email protected]','2024-02-26 15:54:16.974','2024-02-26 15:54:16.974+00','\$argon2id\$v=19\$m=19456,t=2,p=1\$esDS3QCHRH0Kmeh87YPm5Q\$9S+jf+xzw2Hicj6nkWltvaaaXX3dQIxAFwCfFa9o38A');" | yarn workspace @affine/server prisma db execute --stdin

docs/developing-server.md

@@ -29,7 +29,7 @@ docker run --rm --name mailhog -p 1025:1025 -p 8025:8025 mailhog/mailhog
 
 ```
 docker ps
-docker exec -it CONTAINER_ID psql -U postgres ## change container_id
+docker exec -it affine-postgres psql -U postgres ## `affine-postgres` is the container name from the previous step
 ```
 
 ### in the terminal, following the example to user & table
@@ -96,6 +96,12 @@ yarn workspace @affine/native build
 yarn workspace @affine/server dev
 ```
 
+when server started, it will created a default user:
+
+email: [email protected]
+name: Dev User
+password: dev
+
 ## start core (web)
 
 ```