Add custom docker user creation documentation (SYN-4401)

[vEpiphyte]

No description provided.

[codecov]

Codecov Report

Base: 97.17% // Head: 97.07% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (0359f7c) compared to base (a78e108).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2921      +/-   ##
==========================================
- Coverage   97.17%   97.07%   -0.11%     
==========================================
  Files         215      215              
  Lines       42143    42128      -15     
==========================================
- Hits        40954    40895      -59     
- Misses       1189     1233      +44     

Flag	Coverage Δ
linux	97.07% <ø> (-0.01%)	⬇️
linux_replay	?

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Impacted Files	Coverage Δ
synapse/tests/utils.py	94.46% <0.00%> (-1.89%)	⬇️
synapse/cortex.py	96.50% <0.00%> (-0.57%)	⬇️
synapse/lib/hiveauth.py	96.01% <0.00%> (-0.48%)	⬇️
synapse/lib/trigger.py	95.01% <0.00%> (-0.39%)	⬇️
synapse/lib/view.py	97.00% <0.00%> (-0.34%)	⬇️
synapse/cmds/cron.py	97.56% <0.00%> (-0.25%)	⬇️
synapse/lib/jsonstor.py	98.32% <0.00%> (-0.24%)	⬇️
synapse/axon.py	98.73% <0.00%> (-0.15%)	⬇️
synapse/lib/layer.py	96.99% <0.00%> (-0.05%)	⬇️
synapse/lib/chop.py	98.00% <0.00%> (-0.04%)	⬇️
... and 9 more

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