Implement s6-overlay inside the Docker image

[bachya]

Describe what the PR does:

This PR implements the s6-overlay system inside the Docker image.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #306 (7ecfe8c) into dev (c4b1884) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev      #306   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1134      1134           
=========================================
  Hits          1134      1134           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.