Set tags for init and prometheus docker images

[gdbelvin]

gcr.io does not consider 'latest' to be a valid docker tag,
so we must explicitly set one ourselves.

[codecov]

Codecov Report

Merging #1468 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1468      +/-   ##
=========================================
+ Coverage    67.8%   67.9%   +0.09%     
=========================================
  Files          54      54              
  Lines        4026    4026              
=========================================
+ Hits         2730    2734       +4     
+ Misses        894     891       -3     
+ Partials      402     401       -1

Impacted Files	Coverage Δ
core/integration/client_tests.go	84.77% <0%> (+0.23%)	⬆️
core/sequencer/server.go	73.94% <0%> (+0.97%)	⬆️

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