Move main package to root and add Docker image

[pellared]

Why

Little things that improve the ease of use.

What

• Move main package to root. I suggest using the cmd structure only if the repository builds more than one binary.
• Update golanci-lint configuration.
• Build and push Docker container image.
• Refine GitHub Actions workflow.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@8cd93c4). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage        ?   50.00%           
=======================================
  Files           ?        1           
  Lines           ?        2           
  Branches        ?        0           
=======================================
  Hits            ?        1           
  Misses          ?        1           
  Partials        ?        0           

Flag	Coverage Δ
Linux	50.00% <ø> (?)
Windows	50.00% <ø> (?)
macOS	50.00% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
main.go	50.00% <ø> (ø)

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