Update Dockerfile to fix coverage problem

[Psirus]

Ever since #182, there no longer was any coverage information. That's because the codecov script is pulled via https, but curl was missing this certificates package. Anyway, should work now.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (PDE_reviewed@40e23bf). Click here to learn what that means. The diff coverage is n/a.

@@               Coverage Diff               @@
##             PDE_reviewed     #204   +/-   ##
===============================================
  Coverage                ?   67.11%           
===============================================
  Files                   ?      314           
  Lines                   ?    12095           
  Branches                ?        0           
===============================================
  Hits                    ?     8117           
  Misses                  ?     3978           
  Partials                ?        0

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[Psirus]

• The flag -s in the invocation of curl stands for silent. Not sure if that is required, we only copied that from codecov. Now, when everything works fine, it only suppresses a progress bar or something. But when something fails, it also suppresses the error message. I don't remember it anymore, but googling it quickly led to "Do sudo apt install ca-certificates!", which I obeyed, and found that it was good.
• Dunno. Feel free to. Next time it's probably something different.^^