Add proper README to images

[alexandrevicenzi]

Most images currently have a rather small incomplete README or none at all.

Adding a proper README to our images will help users deploy applications quickly.

Containers missing a README:

• [x] MariaDB Server (#1024)
• [x] MariaDB Client (#1342)
• [x] PostgreSQL (#1118)
• [x] 389 Directory Server (#1299)
• [x] Nginx (#1126)
• [ ] Git
• [x] Helm (#1276)
• [x] Go (#1169)
• [x] OpenJDK (#1344)
• [x] Python (#1195)
• [x] NodeJS (#1241)
• [x] .NET (#1420)
• [x] Rust (#1236)
• [x] Ruby (#1238)
• [x] PHP (#1156)
• [x] Kernel Module
• [x] RMT (#1141)

[alexandrevicenzi]

MariaDB Server is done. PostgreSQL is up for review. Starting to work on Nginx.

[Priyankasaggu11929]

I'll work on adding README for Helm.

[dcermak]

I'll take PHP

[alexandrevicenzi]

I'll take Go.

[dcermak]

I'll take the kernel module container

[alexandrevicenzi]

I'll take Python.

[alexandrevicenzi]

I'll take Rust.

[alexandrevicenzi]

I'll take Ruby.

[alexandrevicenzi]

I'll take NodeJS.

[alexandrevicenzi]

I'll take 389 DS.

[alexandrevicenzi]

I'll take MariaDB Client.

The Git container is borked.

[alexandrevicenzi]

I'll take OpenJDK.

[alexandrevicenzi]

I'll take .NET.