docker.yaml: Push docker tag for latest builds on master branch.

[hjpotter92]

What does this pull request do? Explain your changes. (required)
Creates and pushes go-livepeer:latest tag for docker images.

Specific updates (required)

• In case the github action is working on master branch, a docker tag for :latest is generated
• The tag is pushed to dockerhub
• This helps users who want to just run docker pull livepeer/go-livepeer without referencing a specific commit/version.

How did you test each of these updates (required)
This PR!

Does this pull request close any open issues?
Fixes #1352

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2440 (14f2a6e) into master (cb97d57) will increase coverage by 0.01544%.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##              master       #2440         +/-   ##
===================================================
+ Coverage   54.97684%   54.99228%   +0.01543%     
===================================================
  Files             93          93                 
  Lines          19430       19430                 
===================================================
+ Hits           10682       10685          +3     
+ Misses          8155        8153          -2     
+ Partials         593         592          -1     

Impacted Files	Coverage Δ
discovery/db_discovery.go	67.01389% <0.00000%> (-1.04167%)	⬇️
server/mediaserver.go	67.18750% <0.00000%> (+0.46875%)	⬆️

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