Use mainstream runc again now that PR opencontainers/runc/pull/1506 is merged

[juanjux]

• Restored to the ~master branch of opencontainers.
• Tested that it doesn't leave zombie processes.

We should also delete out fork once a new docker image of the server is published.

[codecov]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #94   +/-   ##
======================================
  Coverage    59.8%   59.8%           
======================================
  Files          12      12           
  Lines         836     836           
======================================
  Hits          500     500           
  Misses        298     298           
  Partials       38      38

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