Added apk repository url.

[AutomateAaron]

The the alpine package is in the edge/testing repository, so running:

apk add img

won't work by default, hence the update to:

apk add img --repository=http://dl-cdn.alpinelinux.org/alpine/edge/testing

[codecov[bot]]

Codecov Report

Merging #222 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #222   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files          14     14           
  Lines         777    777           
=====================================
  Misses        777    777

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[jessfraz]

awesome thanks!