Dependencies: alpine-certificates container

Split from #1051 (closed)

Add CNG's alpine-certificates container to configuration for dependencies.io

From https://gitlab.com/charts/gitlab/issues/1051#note_151003041

We're setting ALPINE_VERSION here to the distribution major/minor version, which is akin to 16.04 or 9 for Ubuntu or Debian. An update to the underlying image contents may not have any bearing on the only package in here we care about. This image might be updated without a need for us to build it again. That being said, it might be our best option considering the below findings. ... (see comment for more details)

We should be monitoring based on the availability of the base image tags, using alpine-certificates/Dockerfile. alpinelinux/docker-alpine does not provide tags to check via git.