Expose port 8400 in backend Dockerfile

[djpiper28]

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Name	Status	Preview	Comments	Updated (UTC)
bracket	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 2, 2024 9:36pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.40%. Comparing base (f2f35cb) to head (23cd015).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   90.40%   90.40%           
=======================================
  Files         123      123           
  Lines        3938     3938           
=======================================
  Hits         3560     3560           
  Misses        378      378           

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[evroon]

I didn't add it at first because I think it is a confusing option (it doesn't actually expose anything, it's more of a documentation thing.

See https://docs.docker.com/reference/dockerfile/#expose:

The EXPOSE instruction doesn't actually publish the port.

But it might help to understand which port needs to be published by just reading the Dockerfile, so why not