feat: allow to declare custom db credentials in install scripts

[michivonah]

I removed the hard coded database credentials from the install.sh script and the script included in the manual-installations docs page. Instead I created three new variables (postgres_user, postgres_db, postgres_password) to declare the credentials, which then will be added as DATABASE_URL to the dokploy container.

If no password is defined by the user a random one will be generated with openssl rand --hex 32.

I tested the script on a blank debian 13 server and it worked as expected.

This increases the security of every new Dokploy installation by using a unique password.

This commit solves the issue Dokploy/dokploy#3384 of the dokploy repo.