Add latex-builder image

See merge request omegacen/ci-templates!58

https://github.com/tectonic-typesetting/tectonic/issues/53 has been fixed

Add sonar-scanner-cli Docker image

See merge request omegacen/ci-templates!57

Add conda-builder image and use mambabuild

See merge request omegacen/ci-templates!56

Otherwise conda cannot be found by GitLab CI.

The --cleanup flag is necessary to build multiple images in one
container. But that flag removes everything that's not in
the vanilla kaniko image.

Use the first parent commit for changed files as fallback

See merge request omegacen/ci-templates!55

v5

Closes #8

See merge request omegacen/ci-templates!53

The CI job token has limited access to the API; cf.
https://docs.gitlab.com/ee/ci/jobs/ci_job_token.html