CI: build packages for Ubuntu, Debian & Fedora

Changes only in CI configuration (and a small note in Readme):

• compiles dns-collector in CI on every push and runs some basic tests (just checks that --help and --dumpconfig work for now, i'm not sure if it's possible to actually record some DNS traffic in CI/Docker)
• linux packages are built automatically, but only when a new version/tag is pushed, and it's name matches vX (vX.X, vX.X.X, …) (^v([0-9]\.?)+$)
• rpm/deb amd64 packages are built using FPM, and kept in Gitlab as job artifacts for:
  • Ubuntu 16.04 LTS, 17.10, 18.04 LTS
  • Debian 8 (Jessie) and 9 (Stretch)
  • Fedora 26, 27, 28 (also builds libtrace RPM, since it's missing in their repos)
  • CentOS build are disabled for now (can't find libknot for it)
  • if we need some other distros (Arch?), just let me know, most of them should be possible to add (not sure about BSD, FreeBSD is supported by FPM, but not by Docker and our CI machines)
• i had to create some Docker images with FPM (new Ubuntu and Fedora versions, and fixed Stretch), and made them public on the Docker Hub (and their source files on Github)

Pipeline for a commit push (build and test):

Pipeline for a tag push (build, test, and create packages):