fix CI latest Docker image tagging

[metachris]

📝 Summary

Fix CI overwriting the latest Docker image with both portable and non-portable images (as seen here in the latest release CI action: https://github.com/flashbots/mev-boost/actions/runs/3428613593/jobs/5713128560)

Currently, the latest image on Docker hub is the portable image.

This PR updates the Docker release CI step to do that explicitly.

Also removes pushing images for :1 and :1.4 when releasing v1.4.0 (these image tags seem unnecessary and I'd opt for removing them from Docker hub, otherwise we should also add them for portable).

See also #412

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

Codecov Report

Merging #413 (b83b128) into main (ead84e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #413   +/-   ##
=======================================
  Coverage   65.93%   65.93%           
=======================================
  Files           7        7           
  Lines         907      907           
=======================================
  Hits          598      598           
  Misses        274      274           
  Partials       35       35           

Flag	Coverage Δ
unittests	65.93% <ø> (ø)

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[jtraglia]

LGTM 👍