u-root-container: share the tag parameters between the Makefile and gitlab-ci
Right now, when updating the container tag or the alpine version, we need to update it in 2 or more places, the .gitlab-ci.yml file, the Dockerfile, and the Makefile. This makes it easy for the local environment not to match the CI one.
Let's fix that by sourcing the main parameters (alpine version, tag) directly from .gitlab-ci.yml, using yq.
Closes: #35 (moved)