CI: use Alpine image for [trigger] job

Context

This makes the trigger job use the lighter alpine:3.17 image. This image is used as base for the other opam-repos images and only weighs a few megs.

Before, we used the heavier runtime-build-test-dependencies image (~4.5G). The idea was to precache this image. However, I don't think this makes any sense for various reasons. I can't prove it formally, but I'd be happy to discuss my reasoning further if desired.

Ideally, we'd wanna make sure that we use the same alpine version for trigger as we use in the other opam-repos images. This can be done, but at the price of storing the alpine version in e.g. scripts/version.sh and having to keep that value up to date with opam-repos.

Manually testing the MR

Just trust the CI.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR