dvc.testing: fix lockfile in docker_services fixture

[skshetry]

I wanted to depend on internals of pytest_docker as much as possible, but the lifecycle of it makes it very difficult to do so. (and considering we use pytest-xdist this makes it much harder).

[codecov]

Codecov Report

Patch coverage: 5.88% and project coverage change: -0.04 ⚠️

Comparison is base (a91528c) 92.97% compared to head (f066180) 92.94%.

❗ Current head f066180 differs from pull request most recent head a36cdcd. Consider uploading reports for the commit a36cdcd to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9088      +/-   ##
==========================================
- Coverage   92.97%   92.94%   -0.04%     
==========================================
  Files         459      459              
  Lines       37112    37125      +13     
  Branches     5365     5369       +4     
==========================================
  Hits        34506    34506              
- Misses       2079     2092      +13     
  Partials      527      527              

Impacted Files	Coverage Δ
dvc/testing/fixtures.py	67.08% <5.88%> (-5.90%)	⬇️

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