Update golang Docker tag to v1.13.4

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
golang	docker	patch	1.13.1-stretch -> 1.13.4-stretch

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[codecov-io]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   72.34%   72.34%           
=======================================
  Files           1        1           
  Lines          94       94           
=======================================
  Hits           68       68           
  Misses         15       15           
  Partials       11       11

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