Update docker image generation script + minor update to README

[thehenrytsai]

1. Updated base image used, 18-buster failed Google store vulnerability check.
2. For more easily identifying the docker images.
3. For understanding the tag logic, because current naming is quite misleading.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.08%. Comparing base (f3bdd01) to head (d0c810c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   95.08%   95.08%           
=======================================
  Files          28       28           
  Lines        2948     2948           
  Branches      395      395           
=======================================
  Hits         2803     2803           
  Misses        144      144           
  Partials        1        1           

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[LiranCohen]

Nice! This is definitely useful when wanting to have different versions of the server without having updated the dwn-sdk-js. Will be useful when I wire our DWN instances to have a latest that's different from beta.

Just one sanity comment.

[LiranCohen]

🚀 🐐