test: added docker compose file for testing proxy

[quintesse]

This allows you to easily start up a proxy and a container with a shell
and JBang pre-installed that is only allowed to connect to the proxy.
Includes a README with some instructions.

See #447

[quintesse]

So this is not a solution of any kind, it's just a way to easily test JBang with a proxy!

[codecov]

Codecov Report

Merging #1339 (f36f959) into main (0750e62) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1339   +/-   ##
=========================================
  Coverage     56.42%   56.42%           
  Complexity     1154     1154           
=========================================
  Files            99       99           
  Lines          6151     6151           
  Branches       1020     1020           
=========================================
  Hits           3471     3471           
  Misses         2199     2199           
  Partials        481      481           

Flag	Coverage Δ
Linux	55.24% <ø> (ø)
Windows	55.86% <ø> (ø)

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