Feature/env var command

[acelaya]

Closes #2212

Provide a new hidden CLI command that returns the value of an env var for current env, but making sure the fallback for _FILE env vars is evaluated for that env var.

This allows to call this command from any script in order to get the actual value for the env var, even if that's not a PHP script.

Additionally, use this new command in the docker entry point script, to read environment variables there but retaining the _FILE fallback logic.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.79%. Comparing base (1773e6e) to head (f118ea2).
Report is 10 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #2220      +/-   ##
=============================================
+ Coverage      94.73%   94.79%   +0.06%     
- Complexity      1551     1591      +40     
=============================================
  Files            269      270       +1     
  Lines           5200     5266      +66     
=============================================
+ Hits            4926     4992      +66     
  Misses           274      274              

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