feat: use multi-target-docker-build in ci

Closes #12 (closed)

Description

This MR changes the GitLab CI to use the multi-target-docker-build template to build both the example docker container as the ucam-faas base container that one will inherit from. This means the example.Dockerfile will be merged into the Dockerfile, and we can remove the ./example/gitlab-ci.yml as logic for the child pipeline is all done in the multi-target-docker-build template. I've disabled running tests for the child pipeline, as we only need to do this once.