This adds a GitLab CI/CD component that can be `included` in users
GitLab CI configuration to integrate releaser-pleaser.
Unlike the GitHub Action, this can not easily run whenever a merge
request description is changed, only when changes are pushed to main.
We also switch the container base image from `static` to `busybox`
as GitLab CI requires a shell (`sh`) binary to be present for the image
use to run jobs.
