Add Dockerfile for example-gcp

[AlCutter]

This PR adds a simple Dockerfile for building the example-gcp binary.

Eventually, this can be used for CI/CD testing on GCP.

Towards #7.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.01%. Comparing base (46ec9c2) to head (196dcdc).
Report is 62 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   35.80%   36.01%   +0.20%     
==========================================
  Files          16       33      +17     
  Lines        1363     2824    +1461     
==========================================
+ Hits          488     1017     +529     
- Misses        801     1697     +896     
- Partials       74      110      +36     

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[phbnf]

Ran docker build -f ./cmd/example-gcp/Dockerfile . successfully