feat(ci): Add experimental image building

[iambriccardo]

This PR implements the possibility to create an experimental docker image, this way we are able to release images and clearly mark them as experimental. This applies for images that come from branches that are not main.

[coveralls]

Pull Request Test Coverage Report for Build 19096110303

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 68 unchanged lines in 4 files lost coverage.
• Overall coverage remained the same at 82.367%

---

Files with Coverage Reduction	New Missed Lines	%
etl/src/workers/table_sync.rs	2	77.41%
etl/src/pipeline.rs	4	80.14%
etl-postgres/src/tokio/test_utils.rs	6	89.45%
etl/src/replication/client.rs	56	79.83%

Totals
Change from base Build 19095625550:	0.0%
Covered Lines:	16060
Relevant Lines:	19498

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💛 - Coveralls

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fetch main ref before comparing SHAs

The experimental gate relies on git rev-parse origin/main to detect builds from the default branch, but the preceding checkout only fetches the requested ref. On a workflow_dispatch run for a feature branch, the remote reference origin/main is not guaranteed to exist, so git rev-parse origin/main can fail with “unknown revision” and abort the job even when the ref is valid. Fetch origin main (or compare against refs/remotes/origin/main after an explicit fetch) before running this check so experimental builds don’t fail immediately.

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