Add travis ci, linter, merge shellcheck, fix some issues (#241)

* Quote all vars, remove useless echo/sub-shell, add shebands, fail on unset vars, enable pipefail, formatting

* Add CI including linting via hadolint nad shellcheck

* Update all base images to tag 3.9

* Switch to maintainer labels

* Quote vars

* Remove commented code

* Ignore if the folder exists

lint.sh

@@ -0,0 +1,2 @@
+#!/bin/bash
+git ls-files --exclude='*Dockerfile' --ignored | xargs --max-lines=1 ./hadolint