Update docker images bases to alpine:3.7 and ubuntu:16:04

What does this MR do?

Sets strict version for base image of gitlab/gitlab-runner:alpine (currently we're basing on latest) and updates base image of gitlab/gitlab-runner:ubuntu to 16.04 (currently we're using 14.04 which is a year from its EOL, and in few days/weeks 18.04 - a newest LTS - will be released).

Why was this MR needed?

For alpine image it's not good that we're basing on latest since it can be upgraded at any time without our knowledge. For ubuntu we're using quite old base image.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?