GitLab

The CI currently downloads all go modules in each pipeline step because
go modules reside outside the project directory. Fix this by introducing
a volume for the `/go` directory [1] so modules are only downloaded once per
pipeline using a new `deps-backend` make target.

For completeness, I also included new `deps` and `deps-frontend` targets
and the frontend one is also triggered explicitly on CI where needed.

[1] https://docs.drone.io/pipeline/kubernetes/examples/language/golang/#dependencies