refactor: use os and io packages instead of io/ioutil

[Clivern]

io/ioutil is deprecated so i replaced it according to https://golang.org/doc/go1.16#ioutil

[codecov]

Codecov Report

Merging #2676 (0c4c800) into main (0e964a1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2676   +/-   ##
=======================================
  Coverage   84.68%   84.68%           
=======================================
  Files         102      102           
  Lines        7775     7775           
=======================================
  Hits         6584     6584           
  Misses        977      977           
  Partials      214      214           

Impacted Files	Coverage Δ
internal/pipe/docker/docker.go	88.76% <100.00%> (ø)
internal/pipe/mattermost/mattermost.go	79.45% <100.00%> (ø)

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[caarlos0]

neat, thanks!