Add byo pki cosign support to docker image builds

[vEpiphyte]

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[codecov]

Codecov Report

Patch coverage: 98.63% and project coverage change: -0.11 ⚠️

Comparison is base (ddda6a9) 97.30% compared to head (5ed1c3c) 97.20%.

❗ Current head 5ed1c3c differs from pull request most recent head 0e95c5d. Consider uploading reports for the commit 0e95c5d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3196      +/-   ##
==========================================
- Coverage   97.30%   97.20%   -0.11%     
==========================================
  Files         225      226       +1     
  Lines       45073    45127      +54     
==========================================
+ Hits        43857    43864       +7     
- Misses       1216     1263      +47     

Flag	Coverage Δ
linux	97.20% <98.63%> (+<0.01%)	⬆️
linux_replay	?

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Impacted Files	Coverage Δ
synapse/tools/docker/validate.py	98.63% <98.63%> (ø)

... and 12 files with indirect coverage changes

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