Standardize signature parity testing, add Docker for testing

[kvz]

The Node SDK now has cli support for signing payloads, we can use it to compare our own signatures instead of handrolled Node.js code in this repo:

❯ npx transloadit --help
Usage:
  npx transloadit smart_sig    Read Smart CDN params JSON from stdin and output a signed URL.
  npx transloadit sig [--algorithm <name>]    Read params JSON from stdin and output signed payload JSON.

Required JSON fields:
  smart_sig: workspace, template, input
  sig: none (object is optional)
Optional JSON fields:
  smart_sig: expire_at_ms, url_params
  sig: auth.expires and any supported assembly params

As an aside, I added Docker so it's easier to test this SDK on machines that do not have the Ruby environment set up.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (28060df) to head (dec4c92).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   99.00%   99.00%           
=======================================
  Files           9        9           
  Lines         302      302           
  Branches       39       39           
=======================================
  Hits          299      299           
  Misses          3        3           

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[kvz]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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[kvz]

I could see the asset blew the gem size up. Corrected:

[kvz]

Not sure what is idiomatic here. Should we remove this from the gem completely (but then you can't use it to fully test itself anymore?)