Pre-build docker test image

We use Docker in our development & testing setup to help with reproducibility (see scripts in https://github.com/elastic/elastic-serverless-forwarder/tree/main/tests/scripts/docker).

But we build docker image on the fly, and this create possible reproducibility issues: some dependency may disappear making a build fail due to unrelated occurrences (see https://github.com/elastic/elastic-serverless-forwarder/pull/446, lint check failed due to missing packages that are installed during docker build).

TO prevent the need to quickfix things like we did in https://github.com/elastic/elastic-serverless-forwarder/pull/449, we need to pre-build docker images and host them so the dev/CI can leverage pre-built images without the need to build them on the fly when missing.