Fix image not found issue

[alex-semenyuk]

There is an issue with building docker
ERROR: failed to solve: golang:1.21-buster: docker.io/library/golang:1.21-buster: not found

https://github.com/hyperledger/firefly-ethconnect/actions/runs/7579131739/job/20642836640

Fixed it by using 1.21-alpine3.19 as for example for hyperledger/firefly-tezosconnect#34

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.23%. Comparing base (e3d87b4) to head (c82ec3f).

❗ Current head c82ec3f differs from pull request most recent head 18f0452. Consider uploading reports for the commit 18f0452 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #242   +/-   ##
=======================================
  Coverage   97.23%   97.23%           
=======================================
  Files          59       59           
  Lines        7532     7532           
=======================================
  Hits         7324     7324           
  Misses        164      164           
  Partials       44       44           

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