[docker] Allow to change default sysctl parameters using environment variables

[aivarasko]

When using a higher number of shards current limit of 3M files limit is not enough.

Make these parameters configurable for docker based environments by allowing change using env variables.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3689 (6b5c855) into master (6b5c855) will not change coverage.
The diff coverage is n/a.

❗ Current head 6b5c855 differs from pull request most recent head c794ca5. Consider uploading reports for the commit c794ca5 to get more accurate results

@@          Coverage Diff           @@
##           master   #3689   +/-   ##
======================================
  Coverage    56.9%   56.9%           
======================================
  Files         552     552           
  Lines       62950   62950           
======================================
  Hits        35853   35853           
  Misses      23914   23914           
  Partials     3183    3183           

Flag	Coverage Δ
aggregator	63.2% <0.0%> (ø)
cluster	∅ <0.0%> (∅)
collector	58.4% <0.0%> (ø)
dbnode	60.6% <0.0%> (ø)
m3em	46.4% <0.0%> (ø)
metrics	19.7% <0.0%> (ø)
msg	74.4% <0.0%> (ø)

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[schallert]

LGTM