Updated dbt image name variable

[harsha-stellar-data]

Updated dbt image name variable for this PR https://github.com/stellar/stellar-dbt-public/pull/88

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.

What

[TODO: Short statement about what is changing.]

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

N/A

[chowbao]

I recommend coordinating with @amishas157 before merging this PR. Y'all both have dbt changes that are merged and ahead of the current dbt image