Build docker image on tags without - only (i.e. not for -rc1, -beta)

[metachris]

📝 Summary

Before this PR, every new tag seen by Github would create and push a flasthbots/mev-boost:latest Docker tag.

With this PR, this only happens for tags that don't include a dash, like v1.2.3.

For tags with dash - like v1.2.4-rc1 or v1.2.4-beta1` - only the Docker image for this specific version is built and pushed.

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✅ I have run these commands

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

[codecov-commenter]

Codecov Report

Merging #345 (735b4cd) into main (6a61a56) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #345   +/-   ##
=======================================
  Coverage   81.91%   81.91%           
=======================================
  Files           5        5           
  Lines         680      680           
=======================================
  Hits          557      557           
  Misses         94       94           
  Partials       29       29           

Flag	Coverage Δ
unittests	81.91% <ø> (ø)

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