test: Adds sha to Couchbase dockerfile to address security issue

[tippmar-nr]

Adds a sha to the Couchbase Dockerfile to address https://github.com/newrelic/newrelic-dotnet-agent/security/code-scanning/407

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.20%. Comparing base (b2456ea) to head (f2e988c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3059   +/-   ##
=======================================
  Coverage   82.20%   82.20%           
=======================================
  Files         477      477           
  Lines       30625    30625           
  Branches     3426     3426           
=======================================
+ Hits        25174    25176    +2     
+ Misses       4653     4650    -3     
- Partials      798      799    +1     

Flag	Coverage Δ
Agent	83.26% <ø> (+<0.01%)	⬆️
Profiler	72.51% <ø> (ø)

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see 1 file with indirect coverage changes

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