build(ci): add workflow for releasing main image

[kranurag7]

GitHub workflow that will build and push a new image whenever a new commit is merged on main branch.

I added a commit on my fork's main for testing.

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1155

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (abe0ce5) 80.25% compared to head (5fc16da) 80.30%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1191      +/-   ##
==========================================
+ Coverage   80.25%   80.30%   +0.05%     
==========================================
  Files          66       66              
  Lines        3646     3646              
==========================================
+ Hits         2926     2928       +2     
+ Misses        505      503       -2     
  Partials      215      215              

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[qweeah]

LGTM.

I think what's proposed in this PR can be merged into the https://github.com/oras-project/oras/blob/main/.github/workflows/release-ghcr.yml by changing the prepare job to

TAG=${GITHUB_REF#refs/*/}
echo "ref=ghcr.io/${{ github.repository }}:${TAG}" >> $GITHUB_OUTPUT

This can be done in another PR.

[shizhMSFT]

LGTM