Update git command in build docker image workflow

[amishas157]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with the jira ticket associated with the PR.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated the README with the added features, breaking changes, new instructions on how to use the repository. I updated the description of the fuction with the changes that were made.

Release planning

• I've decided if this PR requires a new major/minor/patch version accordingly to
  semver, and I've changed the name of the BRANCH to release/_ , feature/_ or patch/* .

What

This PR updates the git command to extract sha.
${{ github.event.pull_request.head.sha }} works well when we have a pull request and the workflow is triggered on commit push.
However, for running workflow manually, HEAD helps to extract sha

Why

Currently the build docker image is failing, since that was tested with trigger workflow on commit push

Known limitations

None

[chowbao]

Just confirming; This is the HEAD of the selected branch right like test-gh-actions or master?

[amishas157]

It will be HEAD of selected branch.

[amishas157]

Example: https://github.com/stellar/stellar-etl/actions/runs/10516172476/job/29137804414