[dtest] Fix dtest docker compose config: env => environment

[vpranckaitis]

What this PR does / why we need it:

Fixes dtest docker compose config, renaming env to environment.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #3421 (0952775) into master (0952775) will not change coverage.
The diff coverage is n/a.

❗ Current head 0952775 differs from pull request most recent head 8aa6373. Consider uploading reports for the commit 8aa6373 to get more accurate results

@@           Coverage Diff           @@
##           master    #3421   +/-   ##
=======================================
  Coverage    72.4%    72.4%           
=======================================
  Files        1100     1100           
  Lines      102536   102536           
=======================================
  Hits        74266    74266           
  Misses      23164    23164           
  Partials     5106     5106           

Flag	Coverage Δ
aggregator	76.8% <0.0%> (ø)
cluster	84.9% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	78.9% <0.0%> (ø)
m3em	74.4% <0.0%> (ø)
m3ninx	73.5% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.4% <0.0%> (ø)
query	67.0% <0.0%> (ø)
x	80.5% <0.0%> (ø)

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