Use golang:1.16-alpine3.13 as build image

[jeromefroe]

This commit updates the Docker image we use to build the m3db-operator to golang:1.16-alpine3.13. The current image, golang:1.16-alpine, uses Alpine 3.14 which requires at least Docker 20.10.0 in order to pick up a fix for an issue with the faccessat2 syscall in Alpine Linux 3.14. Longer term we should update CI to use a newer version of Docker but in the interim this commit just uses an image with Alpine 3.13.

[codecov]

Codecov Report

Merging #306 (1cd74a0) into master (f919dd9) will not change coverage.
The diff coverage is n/a.

❗ Current head 1cd74a0 differs from pull request most recent head dd567b9. Consider uploading reports for the commit dd567b9 to get more accurate results

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   76.04%   76.04%           
=======================================
  Files          32       32           
  Lines        2571     2571           
=======================================
  Hits         1955     1955           
  Misses        437      437           
  Partials      179      179           

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[schallert]

LGTM