feat: improve docker docs

[JTorreG]

Proposed changes

Corrects the steps to build the official Alpine-based Agent images.

Adds a note about building images for unsupported OSs

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.12%. Comparing base (6d3944a) to head (a43f068).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #587   +/-   ##
=======================================
  Coverage   66.12%   66.12%           
=======================================
  Files         118      118           
  Lines       13461    13461           
=======================================
  Hits         8901     8901           
  Misses       3959     3959           
  Partials      601      601           

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[mtbChef]

I'd say this is acceptable, but it is missing some info that would make it less frustrating for customers. As an example, the steps to access the container shell and restart Agent after enabling gRPC and REST are missing.

[JTorreG]

waiting on testing of the steps in the docs as per @ninaforsyth 's request