Fix install: use dynamic docker root dir instead of hardcoded one

[boutetnico]

Avoid using a hardcoded path to docker root dir. Instead, fetch the path dynamically and use it.

Use case: folks using a non standard docker root dir.

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Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[hubertdeng123]

Thanks for submitting this!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (f22bc2a) to head (3e57f7e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3064   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files           3        3           
  Lines         203      203           
=======================================
  Hits          201      201           
  Misses          2        2           

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[cbull-86]

Please note that this does not work on a WSL2 Windows install as a default docker install keeps its volumes in /mnt/wsl/docker-desktop-data/version-pack-data/community/docker/volumes.
However, docker info will still report Docker Root Dir as /var/lib/docker, which does not exist.