Add documentation for docker images

[dhurley]

Proposed changes

Add documentation for the following:

• How to build a docker image with NGINX+ and NGINX Agent
• How to build a docker image with NGINX OSS Mainline and NGINX Agent
• How to build a docker image with NGINX OSS Stable and NGINX Agent

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[Jcahilltorre]

approved with suggestions

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.03% ⚠️

Comparison is base (3d9d248) 67.43% compared to head (5d4978b) 67.40%.
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #418      +/-   ##
==========================================
- Coverage   67.43%   67.40%   -0.03%     
==========================================
  Files         113      113              
  Lines       12764    12764              
==========================================
- Hits         8607     8604       -3     
- Misses       3588     3591       +3     
  Partials      569      569              

see 1 file with indirect coverage changes

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