Drop privileges in API server

[matyasselmeci]

Pelican Service:

• Client
• Plugin
• Registry
• Director
• Origin
• Cache
• Other (please give the detail)

The Pelican web/API server runs as root; this means that a security vulnerability would result in the attacker having root privileges on the server, which is not good. This is especially an issue since we instruct people to open the API server to the world.

Have the process running the API server drop privileges. Allow the admin to specify the Unix user and group in their config; by default both should be named 'pelican'. The native packages (RPMs and Debs) should create this user and group if they does not exist. The Dockerfiles should pre-create the user and group with a fixed UID/GID.

Ignore the user:group setting if Pelican is not started as root. Exit with an error if Pelican is started as root but cannot drop privileges (e.g. due to the user not existing). Give a big warning but continue if the admin specifies root:root as the user:group.