test: add basic e2e tests

[fmartingr]

Adds the basics to have proper e2e tests working on the repository:

• A new Dockerfile.e2e used in e2e tests
• A basic e2e test as an example
• Required files for CI to work properly

e2e tests can be run using make e2e and require a working docker engine.

Closes #886

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.33%. Comparing base (a3772c3) to head (e0ed8d3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #816   +/-   ##
=======================================
  Coverage   35.33%   35.33%           
=======================================
  Files          61       61           
  Lines        4078     4078           
=======================================
  Hits         1441     1441           
  Misses       2414     2414           
  Partials      223      223           

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